You can count James Montier, manager of GMO's Global Real Return fund, among those who are concerned about lofty asset valuations.

He told Citywire Global that he has cut the fund's risk to the lowest level since 2008. He has curbed its stock positions, keeping 20 percent in liquid assets, such as cash, and 30 percent in fixed income.

"In 2007 and 2008 we had about 80 percent of the fund in non-risky assets. This has been the first time since that we have had over 50 percent," Montier said. "This is definitely the most difficult time to be an asset allocator. It’s very hard to find value."

The S&P 500 index carried a trailing price-earnings ratio of 21.30 Friday, up from 19.42 a year earlier, according to Birinyi Associates.

The equity positions Montier is trimming aren't speculative high flyers, but rather blue-chips such as Procter & Gamble and Microsoft. "We still see these names as a relatively good option for equity investors, but as we are value investors, we decided to cut them back a bit as they were getting expensive," Montier said.

Procter & Gamble shares have a forward price-earnings ratio of 17.9 times, while Microsoft's P-E is 13.9 times, according to Morningstar.

Meanwhile, Charles Robertson, chief economist of Renaissance Capital, says U.S. stocks are headed for a major tumble, thanks to Greece's debt crisis, China's stock plunge and a likely Federal Reserve interest rate increase this year.

He predicted on CNBC that the S&P 500 index will crater to 1,100 by March 2016. That represents a 47 percent drop from 2,073 Friday morning.

As for the troika of doom, Greece is negotiating a debt restructuring package with its European creditors. In China, the Shanghai Stock Exchange Composite Index has plummeted 25 percent since June 12, though it rebounded 11 percent Thursday and Friday.

And in the United States, economists expect the Fed to raise rates in September or December.

"We're seeing just how fragile it is with that Greek crisis right now," Robertson said. "The second risk is China. That's always a possible problem, which could provoke something much worse. The third is a Fed rate hike, [which has] more unexpected consequences than people assume ahead of time."

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