If you comment on a blog it should be because you want to comment on a blog article and not because you are seeking a free link from a high PR web page. One is a natural thing to do, the other is spam.
it does matter, if you post your comment in a blog with pr, this would help your site to get a backlink and to get pr for your site too, but it should be natural dont spam by putting too many signatures.....
Get your links in blogroll with effective keywords in anchor text, if you've few blogs you can do link exchange easily and that will reflect as one way link for both you and other blogger. Buy few domain name with PR from deleted list and create blog, add fresh content, this is the easiest way to get blog with PR for link exchange.
Yes , with high pr blog will get good position for your keyword, I mean do follow, but if you only in no follow blog, that will be bring you more traffic only, but will not do any effort for your position in GOOGLE.
Blog's PR is not so necessary as PR is not the main parameter for in SEO. It's much better to have a lot of backlinks. Even nofollow links let search engines to index your pages faster and pay attention to your website. And always remember that you should produce relevant comments, no spam. Let's respect each other.