When I'm serging curves, whether inside or outside curves, I have to really slow down. I use both hands to guide the fabric, one in front and one behind. Guide the fabric so it's straightest when the needles are actually going into the fabric. On an inside curve, I do 'straighten' the curve a bit, without stretching the fabric. You can clip just a bit there, not enough that it would be noticeable, but enough to let you sew a straighter line. Remember, that this is the same as you'd do on a regular seam when turning the seam, so you can press it flat.
it's trickier on the outside curves. If there is any extra fabric in the seam allowance, I trim it away before I stitch. Then it doesn't matter where that edge lies before it reaches the needles. I try to keep it straight at the needles, and just after that point.
The main thing is to go slowly though.