To the best of my knowledge a good practice in submitting domains to search engines is having each domain have its own IP. With the announcement by ARIN ([arin.net ]) I am not sure how we are going to deal with many domains having the same IP.
What happens if 30 URLs have the same IP and one of them upsets the search engines? If that IP is banned, out goes 30 domains?
I see no difference in my rankings between domains that are multi-homed [in the hundreds] or single IP, except at AV where I have a banned domain, sites that share the IP get buried in the 90's - 100's.