You really need to focus on a weight that is most comfortable for you and then expand from there. Shoulders take so much abuse from lifting overall. I can tell you that I have had trouble with both my shoulders at different times over the years and its just a matter of not overworking them. If your focus is on increasing your bench, then there really is no need to tax your shoulders another day. I would just add something in on the bench day to allow more rest in between.