Well, the default is 0, so I'm assuming they're all 0, unless I change them. When you renice, it returns the previous and new value.
It's not like anything will conflict if there are several set to -20, the mach scheduler will just let them duke it out in a round robin fashion, just as if they were all 0 like before.
And you really wouldn't run around making everything -20 like ppp or anything like that.
I would avoid reniceing anything with a parent ID of 1, as so much will fall under that process (like Window Manager affects a lot of subordinate processes). Pick a subordinate process (like the dock, or the desktop) to renice, rather than a parent (windowmanager) to speed up the particulars.