I started investing around 1996. My dad (whom I am close to and whom I admire for many reasons) had been bearish since about 1983. Greenspan was proclaiming irrational exuberance. PE10 was climbing into the 30s. I just left everything in cash, some bonds. Bought a little in 02 and then when the market rallied after the tech crash, I 'shorted' by buying BEARX; also VGPMX and VIPSX for the coming (in my view) inflationary crisis. So I really didn't have any stocks at all for the first half of the 2000s.
Then as I started to learn more about investing, about buying/holding/not timing, etc., I was persuaded that being 0% stocks at age 30 was pretty ridiculous. So I moved slowly into the market; but couldn't commit since PE10 had been in a secular decline from around 40 in 2000 to the mid-upper 20s in 2006-07 and I just didn't feel the excesses of the 90s had been washed out of the system. Influential to my thinking was reading Doug Nolan and David Tice (managers of BEARX) and their very prescient views about the credit bubble as manifested primarily by ABSs.
As the market declined, I got up to 35% stocks in January 08, 50% in July 08, and eventually 70% stocks by mid-late 09. I settled in at 55-60% stocks for the long haul, commensurate with my need, ability, and willingness to take risk, as guided by this wonderful forum.
I still firmly believe that valuation matters, at least for stocks. At the extremes, I do not adhere to a strict buy and hold philosophy, but rather to a tactical allocation model.