Deep rabbets only matter in that you have to fill the left over space in with something to prevent your customers from poking their fingers through the backing the first time they pick up the frame! Too much extra work.
3/8 to 9/16 is a typical rabbet depth on so-called standard frames that are deeper at the perimeter than at the rabbet, versus "reverse" frames that are deeper at the rabbet. To make framing life as easy as possible, pick a moulding with a rabbet depth that is about 1/8" deeper than your package, that's just enough to use Fletcher points without fracturing the frame, but not so deep that you will get finger poke-throughs if you don't add extra filling. There's a lot to be said for picking moulding that is easy to work with, and I have seldom spoken wiser words than that.
But personally I don't use backings. I mount canvas to Gator which is essentially a hermetically sealed package so I don't need no stinkin' backing. Nobody seems to mind, and it lowers the price a bit.