Are you talking about OS X in particular, or any apps?
Not that it really matters...I do report some bugs that I run across, but I tend to be better about it for third-party apps, since I feel like they often need more help identifying bugs. (But then again, if everyone thought like me and didn't submit a good portion of their OS X bugs, Apple might not know about them either...or at least not consider them a priority for squishing.)