I recently purchased a set of 5mm spacers to move the rear wheels out slightly due to rubbing of the inner plastic fender. I did receive 10 bolts with it, but just wondered if I could stick with the stock wheel bolts.

 

You have to use the longer bolts.

 

I forget the rule of thumb, but I believe it was something like a minimum of 6 threads engaged into the hub.

I see no reason (other than aesthetic...sometimes bolt colors don't match front to rear) not to run the correct length bolts, especially if you have them in hand. Much safer to increase your thread engagement by the same amount that the spacers decrease it.

Full disclosure: I ran stock bolts when I had 5mm spacers on my e36 BMW, no problem. They did not come with bolts, and I had 6-7 turns on the wheel bolts, so I ran it that way. If they would have come with longer bolts, I would have used them.