For the baseboard in the picture below, would you recommend coping, mitering, or butting the internal corners? The profile is about 7" high and .5" think. I intend a paint finish.
This is a matter of opinion and prior experience. I've done plenty of this profile, and mine is that coping will yield the best outcome. Inside miters are difficult to make tight while keeping them aligned, and they don't tend to stay tight.
The right round file, sanding rod, or Dremel barrel would make it fairly simple. The fact that you're painting means you can fill any imperfections.
- Cut the piece to length1 at 45°, as if you were going to miter it.
- Flip the piece and cut at 90°2 down the flat face at the corner created by the initial cut.
- Optionally do a rough cope of the bead with a fine-bladed coping saw.
- Finish the bead to the initial cut with a suitable round file (or sandpaper around a dowel or drill bit).
1) The beauty of a cope is that you can force it in a bit for a lasting, tight fit. Add ~1/16" to your length, then flex the piece out at the center while you set the ends. You can't do this with a miter as easily--the two pieces tend to shift against each other.
2) Back-cut slightly (1-2°) to be sure the front edge remains tight. Drywall corners often curve into the room a bit at inside corners, meaning the ultimate angle is somewhat more than 90°.