Most (all?) programs want at least one LOR from an EP. If you only have one, it should be from someone that works with residents and has a role in resident selection. Personally, all three of my letters were from EPs; two from an away and one from my home rotation. If you choose to have LORs from other specialties, make sure to ask them to address qualities unique to EM--ability to adapt, multi-task, work independently, etc, as these are things that PDs will look for.
In terms of away rotations, I think there are really two reasons to do them: either you're rotating at a place where you really hope to match, or you're rotating at a place that's different from your home program (county vs. private vs. community, etc). For the latter reason, it makes the most sense to do this early. If it's a place where you really want to go, then December-February aren't too late, as it will still give you and the program a chance to evaluate each other. In fact, you should be a few months more "clinically mature" in Dec/Jan/Feb and may perform better as a result. Also, they're likely to have non-EMers rotating then, so you may compare favorably to other students. Obviously, you won't be able to get a letter, but if it's your numero uno, their opinion of you (and your's of them) is much more important than a LOR.
