I lap to make room for bedding. Lapping also gives the ring surfaces a rougher surface for bedding adhesion. Bedding could be done without lapping but you can end up with "hot spots". By that I mean if you only had say 30-40% contact on the bare rings to begin with, then that's roughly what you will still have when the bedding shrinks because those contact areas on the scope tube will have a much thinner layer. So when the bedding shrinks over time, you might end up back at square one. If you lap the rings generously before bedding, then you have a good even layer of compound between the tube and rings at all points which, in theory, should maintain even contact on the tube when the compound shrinks. Therefor I would not bed without lapping.
I also recommend using JB Weld for bedding scope rings over MarineTex or Devcon. MarineTex has the least amount of shrinkage by far over time, but it also has a lower friction coefficient than the others (More slippery surface). JB weld maintains a better gripping surface after curing. If you have used both MarineTex and JB then you'll know wat I'm talking about. Devcon kind of sits in the middle. Not as good of grip as JB, but also shrinks quite a lot over time. I use MarineTex for receiver bedding because of how well it holds dimensional form over time and JB weld for scope bases and rings for it's greater gripping ability to avoid slippage and movement. If scope tubes had recoil lugs built into them, then I would surely use MarineTex for base and ring bedding.