Does it matter?
I think you should care if you are going to setup a blog farm for link building/SEO purposes, but if you are gonna setup a bunch of blogs to display adsense and promotte clickbank products it doesn't really matter (unless you get de-listed, then I think all the blogs in the IP will be affected). It will be best if you have a unique domain for each blog but if you think it is a waste of money, go with some free domain like co.cc, etc.