I believe it is an absolute must, definitely. You don’t want to be sharing an IP due to the following reasons:
1) IP could have been previously blacklisted by Google or another search engine.
2) Potential to get blacklisted in the future after you’ve spent a lot of time SEO’ing your site.
3) Hide your sites from competition. If you host multiple sites on one IP, it is very easy for your competition to reverse-lookup that IP and find all of your sites.
I’m sure there are other reasons that I can’t think of immediately.