Bismarck,
My guide works like this:
1. If I have a different SLA from the main site collection, it's a new site collection.
2. If I have totally different content types or content types that want to work against the core of the main site collection, that's a different site collection
3. If I expect wildly different content and traffic patterns and I anticipate possibly having to separate it off onto its own server, that's a different site collection
If you're doing proper database maintenance, having a lot of site collections on disparate databases could be a nightmare for your DBA. That's what should keep you from automatically sprawling site collections.
There are probably some better guides from veteran consultants, but that's my $0.02