For what it's worth, when I discovered the site last year I started working through the Overcoming Bias archives linearly, and I went ahead and commented on those posts when I felt moved to.

Occasionally people would respond; more often they wouldn't. Which is true for my comments on current posts, as well. And others'. So I'm not sure there's too much difference.

All of which is to say that I endorse overcoming the sense that commenting on old posts is too silly to do.