In my experience, with plastic you're better off sanding on a flat surface for mating parts.
I do use them for wooden models, and they are great. You may also want to consider a plane with a razor blade, which is a lot cheaper and does well for thin cuts (and I do use for trimming plastic parts, but more often sheet/stock when scratchbuilding):
I have a mini-plane that, is made by Stanley. The thing is probably about 3 inches long, an inch-and-a-half wide, and about an inch tall. A finger-tightened screw holds the blade in place, and adjusts the depth of cut. I originally bought it for fine-tuning joints in my woodworking projects, but I've found it has a multitude of uses.