- Even if you just put in 15A breakers and receptacles now, does it pay
to pull through #12 wire just in case you ever later want to
upgrade?

Yes, everytime you use an outlet you are heating your home with
electric heat. The heaver the wiring the less heat. Codes do not
factor in effeciency in any way so you should. The heaver guage pays
for its self very quickly. In 18 years my company has never run a 15A
line to anything.