I have a tendancy to do something simillar, so I see no problem with your doing it. I like to go through the older posts and try to find nuggets of truth that I missed the first time. A sort of random approach though, rather then by author.

If you think about it, you are not introducing any negative effect this way. You can only vote for a post once, regardless of its age, therefor if you find an older post that deserves a ++, by all means vote for it! (And likewise, if you find bad posts, vote them down!) The real bias here would be your choosing to abstain on a node because you want the author to be promoted despite the crummy post. However, as most people here seem to vote ++ or not at all, the bias is negligable.(Compare the rankings of the Best Nodes with those of the Worst Nodes. There is a substantial difference in absolute value.)

So happy promoting.