There seem to be many different approaches, but my love for everything KISS leaves me with 6 psi, then maybe 15 psi, then up to as much as I can get, about 24 psi.
Good question about the pressure build up, and I guess it would be a matter of experimentation. I don't think that if primary was really done you'd ever have to worry about safety, as the kegs are rated to 130 psi and the release valve in the lid gives out before that. All you'd be worried about would be slaughtering the yeast, as they will die off very quickly above 24 psi (so I've read on the internet, where everyting you read is true.) You might be able to get away with just pulling the keg off the spunding manifold at the appropriate time and checking the pressure regularly, and if you don't care about the potential mass murder of billions of little hard working yeasties, it would likely not be a problem.
I dry hopped that Light Summer Ale that we were tasting in the second Keg after getting the beer off the yeast.
JQ