I usually wait for Denis to let us know when to upgrade to the next SP and WP version.. Anyone know?
SP for any stable release. WP as you see fit normally, I discuss that with Denis as well.
Right now SP is compatible with WP 3.0.1 and WP 2.9.2 so you have the choice. The memo on WP is "3.x only if you need it". That's how I take it for my own and client sites as well. Have been doing good with it so far.
Keep in mind that WP 3.0.x might be incompatible licensed with some plugins and themes it was compatible to before 3.0.0/3.0.1 and will be compatible again after the next release - be it 3.0.2 or 3.1.0 .
Short version of what Hakre said: :-)
I'd like to stress the importance of that second point: