Stocks used to be cheap. Now they're not.

That's how some analysts see it in the context of recent valuations. The question now is whether investors should be concerned—and if so, how should they alter their market strategy.

The most popular valuation metric for the market, the 's forward price-to-earnings ratio, is sitting at a seven-year high, and is mighty close to 10-year highs.

The S&P 500 is now trading at 15.88 times analysts' bottom-up estimates of what companies will earn over the next 12 months, according to FactSet. That's the highest the S&P P/E ratio has been since 2007, and nearly the highest since 2005. Back in July 2007, it ticked only slightly above that to 15.93, as stocks traded just about at a peak that went unsurpassed until 2013.

S&P 500 price versus forward P/E ratio (FactSet)

The recent uptick in valuations comes as stocks rise, albeit mildly, to a fresh record high in nearly every session. And while the P/E numerator is rising, the denominator is falling, as the recent crush in crude oil prices is causing analysts to meaningfully reduce their profit projections for energy-exposed stocks.

Back on Sept. 30, analysts were looking for the energy sector to show earnings growth of 7.8 percent in 2015. Analysts are now looking for a 1.4 percent decline in profits. That huge reduction has helped bring the overall growth-rate projection for S&P 500 earnings down from 11.8 percent to 9.7 percent, according to FactSet's senior earnings analyst, John Butters. The market's P/E ratio has risen accordingly.

The question now is whether investors should become nervous. After all, with the benefit of hindsight, we now know that nervousness was certainly warranted back in mid-2007.

"Absolutely you should be nervous," opined Jim Iuorio of TJM Institutional Services. "Whether you're looking at a five-year or two-month chart, you can see that we've pretty much been on a one-way street. Do I think price-to-earnings multiples are the be-all and end-all and we should just trade on that? No. And I think that you should still be bullish. But I think you should be bullish in a smart way."

On the whole, the market certainly does not seem too concerned about valuations. Fund flows continue to be strong, with $29 billion flowing into stock mutual fund and ETFs in the two weeks ended Nov. 12, according to Bank of America Merrill Lynch. And after spiking in mid-October as the market plunged, the CBOE Volatility Index is back to historically levels—which shows that investors aren't too interested in betting on, or insuring against, large downside moves.

Indeed, according to a Tuesday note from Simon Maughan of OTAS Technologies, while valuation metrics point to a short-term peak, "implied volatility continues to tick down. ... At the moment we are watching for a trigger for another downturn, but have not seen one."

"The doomsayers will be right at some point, but implied volatility isn't telling me that it's going to be any time in the next three months," Maughan added in an interview.

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Still, some say it's time to reassess portfolios, and perhaps take a step back from overly bullish strategies.

With valuations much higher than they've been of late, "I don't think being in stocks is the slam-dunk it was a couple of years ago. So we're being more cautious in equity allocation, trying to emphasize quality and stability rather than just growth and a rebound off the bottom," said Curtis Holden, senior investment officer at Tanglewood Wealth Management, a Houston-based firm that manages $830 million in client assets.

"One part of the market that has looked expensive has been small-cap stocks, so we're positioning our portfolios toward larger caps, toward the higher-quality part of the spectrum," he said.

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And as far as individual investors go, Nicholas Colas of ConvergEx notices that many who may have been badly burned in the prior crash are "putting aside a permanent cash buffer in their personal portfolios, to give them a safety net of as many years of retirement income as they need."

For more active investors, other opportunities present themselves.

Brian Stutland of Equity Armor investments recommends allocating a portion of one's portfolio to volatility products, which tend to move inversely to the broader market, and rise during bouts of uncertainty.

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And in recent days, Iuorio has actually taken advantage of the low level of implied volatility by taking some of his stock holdings and trading them out for call options. Calls give one the right to buy a stock for a given time and price, so offer upside exposure without the downside risk of a stock. (Instead, one must pay for the options contracts themselves.) And at the very least, Iuorio recommends that investors rebalance their portfolios, so that they are not more heavily levered to the market simply because stocks have gained so much in value.

Even those bullish on stocks are advocating caution. Matthew Tuttle, whose Tuttle Tactical Management actively manages portions of portfolios for advisors, says that between the global central bank actions that have reduced the attractiveness of nonequity assets, a seasonally strong period of the year ahead for stocks and the underperformance by money managers who will likely look to buy stocks heavily, it's "all systems go right now."

But he adds that "you've got to keep your head on a swivel. You can't just buy the S&P and hold on to it anymore. You have to be ready, when you start seeing the signs, to get to some sort of safety."

Still, even though valuations are at historically high levels, if one believes that future revenues will continue to outpace expenses, stocks may still offer an attractive value proposition in the long term.

"Yes, stocks aren't cheap, but neither is a Rolex, and people still buy that," Colas said. "You're getting what you pay for. And with stocks, you're getting a piece of society that is overearning prior cycles.

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