I notice a lot of custom roms tell you to wait ten minutes after the firstboot before touching anything and was wondering why it is so.
I've never seen that particular caveat, but the most likely things I can think of:
The worry may be (particularly for first-run scripts) that if you start putting an interactive load on the device, the OS will kill those scripts that are running in the background.
In addition, interactive performance will suffer while all these things are going on. ROM authors would probably much rather have you go have a snack and come back to a phone ready to go rather than have your first interaction involve poor performance until the OS is "warmed up".