What? Balance the front with the back? I don't think you'll find that cycling overdevelops the muscles in your back.

Opinions vary on the usefulness of weight training for cyclists. Cycling is primarily an aerobic activity, leg strength is rarely the limiting factor, but plenty of very experienced and successful racers say they see benefits from doing strength training in the off-season.

One thing that is worth thinking about is the demands on different leg muscles. Cycling and running use those muscles in very different ways. Personally I run once a week, not because it helps my cycling (it may even hurt it) but just to give those underused muscles a different workout. Plus, cycling tends to shorten the hamstrings, so try to be diligent about stretching in order to keep them, and the hip flexors, loose. And core stability is helpful in any sport, so core exercises - squats, push-ups, planks, burpees - are all good.