Excellent caution with one caveat. I have a poker site (purchased) with a ton of high pr comment spam links to it, but the PR of the poker site is miserably low...
What it tells me is that google can easily figure out blogs that allow spammy unmoderated comments (by looking at the 'buy viagra' comments posted), so it can discount those blog pages.
If I am correct, it is better to comment on pages that are "under the radar" i.e. are not showing any "buy viagra" links.
This is my personal experience. Comments are appreciated.
P.S. I gave 'thanks' and a 'rep' to OP.