Well, it starts out if you mean right now or when times are "good."
Frankly speaking, P/E ratios aren't my game, but given that they were quite high before the stock market crashed, I'd maybe consider them a little more right now as a decent indicator. There are some folks that live and die by P/E ratios. I think honestly you've got to look at company vitals and look for the infamous "recession proof" stocks for the short term or simply sit on others for the longer term. I'm not one to give advice on technical trading, but I like to consider profit, cashflow, debt, etc. and take a look at the intangibles such as quality of leadership and also be familiar with the market I'm getting into. (IE: You don't want a company that's under the debt water.)