One thing to watch out for with E-commerce sites is the infrastructure to support the necessary level of security. For instance, you may want seperate IPs (multi-homed machine) when using SSL. If all the databases are on the same machine, you should be taking steps (e.g. PHP's safe_mode) in order to keep customer's sites isolated from one another.

That said, like others have noted, it really "just depends". One thing that may be worthwhile is to put on a good stat/logging package so that you have the information necessary to make the right choices, so you know in advance if one site or another is starting to grow and will soon need a larger metaphorical pot.