Modeling Electric Heaters

I assume you mean for the baseline system. System 1/2 are for residential buildings only. For some heating only spaces you can model a System 9/10 which would include fan power. There is a LEED Interpretation that says you can do so for LEED 2009 projects.

You're required to model cooling in the proposed model even if the proposed design does not have cooling equipment. In this case, you'll model cooling identically between the baseline and proposed (efficiency, fan power, etc) but you'll still get some cooling savings from improvement envelope and lighting. If a proposed electric heater is modeled as a PTHP with cooling, that's likely the reason, and they'll model the heating source for the PTHP as electric resistance (or, in the case of eQuest, model it as "Not Installed" and switch on "Electric Baseboards"). The logic is: over the lifetime of the building, there's a chance cooling equipment might be installed. Including cooling in the proposed model should help optimize the design. 

All this is based on 90.1 2007. The 2010 version introduced systems 9 and 10 for spaces not intended to receive cooling (like the stairways you mentioned). In this case, no cooling is modeled in neither baseline or proposed. Like Marcus said, these systems have fan power in the baseline, and you should show some fan savings in the proposed model from switching to radiators.