In a well fitting rabbet there is no side to side or up and down. I've been using mine like that for 20 years or more and I know that Mike from the big D does the same and has for a long time. I'm sure there must be lots of other members that do the same as us.
From a physics point of view, the pressure against the bit does form a lever arm that would want to press the trailing edge of the plate down while lifting the leading edge up, but 1.- because the bit usually insn't very tall it is a very short lever arm, 2.- in most cases the fence is clamped down on top of the plate, and 3.- in all cases you are pressing the wood down onto the plate until the wood goes past the bit, at which time it ceases to be a problem.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.