I never check bags and gate agents tell me that I go to the head of the volunteer list when traveling internationally. If they use my seat they don't want to have to go to the time and trouble of pulling my luggage off overseas flights.
In April I answered the call and volunteered to give up my seat on Delta (jfk to dca). Another pax was not eligible for consideration as a volunteer because he had checked baggage and this was the domestic portion of an overseas trip. I originated overseas too but I had no checked baggage.
I believe that if your itinerary is purely domestic it makes no difference if you have checked baggage or not.