I dont think heat pads are necessary (unless you're in Alaska). I think you should assume that the rabbits will use it as a litter tray tho, sorry, so best not unless it's easily washed.
My two have a small soft dog bed. Muppet was quite happy that it was a bed, not a litter tray. Sooty thinks it's a litter tray though... :roll:
(I'm assuming you mean a soft bed, that goes indoors, rather than something substantial for use outdoors.)