Self leveling underlayment - should I tile first?
personally i don't tile atop plywood.  i'd pull the 3/8 then concrete backerboard would go down first over the subfloor.  then i'd shim the shower.  if the shower is heavy i might then give it a week to let it compress the wooden shims and re-test for level and re-shim if needed.  then i'd tile.  some people are less retentive than me though and that's okay too : )