The reason for using multiple hosting accounts is when you do a private blog network, you do not want 50 sites on the same IP pointing at another site on that ip -- it makes it real obvious what you are doing. If you have (as an example) 4 sites in different niches on one ip its not a problem. Many hosting companies may have hundreds or even more sites on one IP.
So the rule I use is if your using sites for a blog network, hosting is so cheap = spread them out and cut the risk. I usually have 10 or more websites on one IP, but they are not related sites, dont interlink, and all have domain name privacy as well.