It is perfectly safe to do system update on all component, theoretically including not only software update, but operating system update.
Software is always ok because it doesnt mess with your install.
Os is ok because, theoretically, because of chameleon and the Extra folder. Here is where you should put all kexts that get replaced during an update (audio, video, whatevers) - also disabler kext and appledecrypt prevent problems, but I really dont how to explain. That being said, I've had some problems installing kext to the Chameleon Extra folder, some kext loaded, others didn't (dependencies etc) - and I have such a stable system I really dont want to mess with it right now. I'm just riding the waiting waves for Snow Leopard
Anyway, from what I have read, Chameleon 2 rc 2 might be the holy grail for retail install method... got to get into it deeper though...
Stiwy - update software no problem. If you are having OS issues with something, and an update fixes it, go ahead. If not, just leave it. This is 10.5.7, the next revision will be out very soon, and then what, in 1 year they release 10.5.9 with some seriously deep - Leopard wont be much better than it is already, just mini-bug hunting from now on...