As part of finishing my attic (insulation, drywall, etc.), I'm going to add receptacles and recessed ceiling fixtures. Currently, there are two 12/3 cables up there that are capped off, run by a previous electrician. These run down to the main panel to two 20A breakers labelled "Attic spare 1" and "Attic spare 2". If memory serves (I'm not home at the moment), each one is connected to a single-pole breaker, so I'm assuming the red wire is capped off in the box. I'll check this when I get home.
Regardless, my question is the following: for each 12/3 cable in the attic, should I extend it via a junction box using 12/2 cable? In other words, 12/3 going in (red wire capped off) and then 12/2 coming out? I have no need for split-wire receptacles or 3-way switches, so does it make more sense to just run 12/2 from the junction boxes? Or is there some reason that I should continue to use 12/3 throughout the room, to all the receptacles and fixtures? I don't have any 12/3 or 12/2 on hand so I want to know which I should plan to buy. Thanks.