The number of times the burner cuts in and out is irrelevant; the only thing that matters is how long the burner is on in total. Answer: keep it as low as you can live with.

More important to your heating costs is how much you let out the door (or the window or all of the other holes and cracks). If your water tank is not insulated, insulate it. Replace the weatherstrips on all your doors and windows. Caulk every crack or pin hole. If you can, double the amount of insulation in your attic.

Think of your house as a slightly leaky pail, leaking your money — i.e. your expensive heated air. Every pin hole you plug is money kept in your pocket.