I also like to mix things up with no follow links for two reasons:
1) If you're commenting on a popular blog or forum, you'll get traffic if you post on topics that are hot - people will scroll through all the comments and posts and check out your site, and those people will be very interested in your subject matter and highly targeted and
2) You still want to look "natural" to Google. If everything regarding your site appears overly optimized and not organic, Google will know this. Think of it this way. If I bring out a site about Britney Spears latest breast implants, in the "real" world of organic linking I'd be getting links from all kinds of sites - not necessarily just celeb sites. This is how news or hot topics spread in reality - they cross boundaries, and sites link in to your site using all different keywords and anchor text. And you'll get links from everything from pet lovers sites to gossip sites to gadget geek sites.
When we overly optimize or try to control a site so that the ONLY links coming in are from carefully selected places where the niche exactly matches our keyword - that's not natural. Google likes things to be organic or else it raises a red flag.
So it's good to comment on some forums that are highly related to your niche and keywords - plus it's just as good to comment on irrelevant forums or forums that cover more generic topics. Same thing for blogs.
While certainly you need SOME of your backlinks to be from highly relevant sites, don't go to extremes. A "social vote" for your site coming from a blog or forum unrelated to your keyword still carries great power in the overall SEO snapshot that Google takes for your site.
Dan