I use Udyr quite a lot in the jungle and usually start build boots+3 then Heart of Gold. After that should I build Philosopher's Stone for more gold, or go straight for my normal build (Wit's End / Randuin's Omen, etc.). I've seen both Philosopher's Stone and non-Philosopher's Stone done.
Udyr's farming is so good that you really don't need a Philosopher's Stone to keep up with others, just the Heart of Gold will do. What I usually do is go boots+3 pots and then Heart of Gold, Wit's End, and then Randuin's Omen. Once there, you can build whatever you really want (for me it's usually Fratmogs).
I don't find the Philosopher's Stone really necessary on any other junglers. If you're getting counterjungled though, the extra gold is nice.
Basically, if you're jungling like a boss, forget about Philosopher's Stone.
If you need the extra boost, then Philosopher's Stone is awesome.
:) Have fun in the jungle!