I always do that, and I would have to think most good poker players do it at least to some extent. You aren't making money if you fold every time the board's a little scary (i.e. three suiteds and you don't have the flush; board pairs or hits trips and you don't have the third or fourth, but do have another pair; three to a straight; and so forth). There are good reasons to fear these boards of course, but you can't be over-cautious and always fold against someone betting on a scary board. This is more true the smaller the field is. i.e. you're in heads-up, the flop comes A23, and you hold an A. What are the odds your opponent flopped the straight? You can't be too afraid.