It is safer to spread out the hosting amongst several reputable hosting companies on different IP address's. This will reduce the possibility of your sites getting either banned or downgraded by the search engines. Some people are paranoid so they prefer their sites to be hosted on different IP blocks couple of reasons are :-
1. If you use either free or very cheap low cost hosting companies you could be sharing with some nasty or undesirable web sites. If these web sites end getting banned or black legged by the search engines for taking part in "illegal" search engine optimization techniques they may blacklist or down grade the IP address which will effect you as well, even though your site is fine.
2. If you use the same hosting company for many sites and then start linking these together to try and improve the page rank you may get into trouble. It is not that difficult for the search engines to work out the IP address of the sites linking to you. If it finds that they are all on the same IP address this may be considered as link spamming. After all what are the chances that a group of genuine links from a number of different web sites are all going to be hosted on the same computer - not likely!.