PACU scheduling question

I am a PACU staff RN charged with doing the next schedule in our unit. It is the first time for me. We have 14 FT/PT nurses and 7 contingent (or casual) RN's. Our 11 daily shifts are staggered from 7am to MN, and most nurses prefer earlier shifts. Currently, there is a policy of "bumping" in place. This means that if a nurse working the 7:30am shift asks for a day off, all nurses working later shifts that day are automatically bumped to the next earlier shift if that if their prefernece. This leaves the later shifts for contingent nurses to fill.
My manager has told me that she feels FT/PT nurses deserve preferential treatment because she feels their commitment to the unit is greater. Contingent nurses earn the same pay without benifits, but do not have to do call.
I don't agree with this policy. I think the contingent nurses should fill the available shifts whether they be early or late. Before I go to battle on this, I'd like to hear other opinions.
BTW, as you may have guessed, I am contingent!