I tried CCW rotation again last night and it does seem to get confused every once in a while - either decrements by 2 or sometimes even increments. Doesn't happen often enough to be too much of a problem, but I would like to make the code as efficient as possible.

Not sure what else I can do in the ISR, though. If I only need one edge to check the rotation direction then I could do as Henrik suggested and turn off 2 of the edge interrupts; that way, it would only hit the ISR twice. I could do the RotEncDir determination on the first interrupt then set a flag to ignore the 2nd interrupt, then reset those after the 2nd.