The only reason primer is recomended before bondo is the primer will protect from rust better. If you were to get a nick to the metal right next to the bondo, water could travel under the bondo and rust a hole where the bondo is. Its pretty unlikely to happen and if you keep nicks like that repaired you wouldn't have a problem with rust anyway. Its the person that once has a vechile is painted never touches it again that gets in trouble. If a nick occured on a vehicle that was epoxy primed and not tended to the rust would be isolated to the nick and not spread.
Bondo is not real good to work with in winter either. Unless you can get your basement 75 to 80 degrees you should allow a lot more drying time for the bondo to cure. Otherwise you can fix a dent, sand it flat and when it cures it can shrink on you bringing the dent back.