Valuation doesn't matter when a stock has momentum, BTIG Chief Global Equity Strategist Dan Greenhaus said Friday.

"Valuation, while important for investing, is not necessarily a timing tool," he said. "Just because something's overvalued doesn't mean it can't become even more overvalued."

On CNBC's "Halftime Report," Greenhaus said the importance of a stock's valuation depends on the investor.

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"If you're chasing growth, then Twitter could be attractive," he said.

Twitter, which went public Nov. 7, has yet to report its first quarter of earnings, Greenhaus noted.

"So, to some degree, this might be more justified than we think," he added. "But again, to apply this to the market as a whole is something I wouldn't do."

Shares of Twitter have jumped 68 percent since Dec. 1.

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Josh Brown of Ritholtz Wealth Management said that there was a clear sign investors should watch.

"You'll know when valuation matters when there's good news and the stock fails to rally on that, that's when all the buyers are in," he said. "That's exactly what the signal is—good news, and the stock drops or it doesn't rally. That'll tell you valuation's starting to matter."

Stephen Weiss of Short Hills Capital said he generally avoids the valuation factor when evaluating a stock.

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"I don't buy valuation, unless I see a catalyst kick the shares higher. Similarly, I don't short valuation, unless I see a catalyst kick the shares lower. There's no catalyst here to get them lower. You could've made the same short valuation case on Twitter, frankly, on the IPO at $40 as you can at $50, $60 and $70," he said.

"Now, it resonates more at $70, but what we forget often is that momentum is, like it or not, an investor class, an asset class, a discipline, whatever you call it, and that's what's taking hold of this stock and Facebook and others."

Skybridge Capital's Anthony Scaramucci said that positive sentiment among retail investors was largely responsible for momentum in certain stocks.

"Retail likes things like Twitter and Facebook and things like that," he said. "And they like growth."

Scaramucci said that even Twitter's first quarterly earnings report probably won't affect sentiment.

"Earnings will probably not be what people expect, but I don't think it will dampen the appetite for this name," he said.

— By CNBC's Bruno J. Navarro. Follow him on Twitter @Bruno_J_Navarro.

Disclosures: Greenhaus, Weiss and Scaramucci do not own any Twitter stock.