All Thumbs,
Benifical is not a word I normally use to discribe grubs. Actually they can be quite a bothersome pest, depending on what type they are. A lot of grubs eat roots in their larva stage and when they grow up and turn into beetles they do can do damage to the tops of the
plants. Larger animals such as racoons and skunks will tear up a yard in search for them while in the grub stage, not to forget that they attract moles like a magnet. While I am not familiar with the ones that you may have in Cali. Two of the most common in these parts, (Wisconsin) are the European Chafer, and Japanese Beetle. There are others that are pretty much harmless, and don't bother anything. While they are in effect good for the compost pile, they may be harming the rest of your
plants after they change. The bad part is that there is now way to treat the pile for only the grubs. Any pesticide that you add to the pile will also hurt the benifical bugs that you need in there. I would not worry to much about them being in the pile, as they are most likely thru out your yard anyway.
A great way to get rid of them from your compost pile would be to find some kid that likes to fish, and tell em that you have a source of free bait!
Give em a pitch fork and let him dig them out. This way you get yourself out of the chore of turning the pile too.