I would like to run cable for an in-wall heater but not actually connect to the heater right now, I just want to get the cable run and connected to the sub-panel (along with all the other cables that we are running). That way, I don't have to bring in the electrician again later on. So can I run cable from a sub-panel to a junction box and put wire nuts on the three wires and leave the junction box accessible - covered by a box cover but not covered with dry wall? The work will be in a room that we will be renovating in the next couple of months. Does NEC allow this kind of thing?
Putting a box in at the end of the future run, terminating the wires in nuts, and putting a cover on it is standard practice for a "future expansion" outlet of any sort -- putting a label on the cover indicating what it's intended for is a nice touch, but by no means needed by Code. In fact, you cannot "bury" the box behind the drywall -- it must remain accessible for later use/servicing.