I carve in the dead of winter.
Reason is, carving, especially with power tools can vibrate the trunk, the movement of the trunk can, does, will usually break fine feeder roots. The more you jostle the trunk, the more the trunk vibrates, the more roots you are likely to break. And the roots you will break are the fine, tender feeder roots.
So in the dead of winter, I bring in my tree to carve. Do my carving while the root ball is frozen solid, and get it back outside before the roots have a chance to thaw. This way, vibration and breaking of fine feeder roots is minimal.
However, in New Mexico, and through much of the south, you simply do not have roots frozen solid. In which case I would time carving to happen just before the normal times for a spurt of root growth. If the jostling, vibration and banging around of the trunk breaks a lot of fine feeder roots, you want to do the carving at a time that is just before a flush of new root growth. So late winter, and late summer are my normal repotting times, I would carve just before these windows of time open up for me.
This year, the media in most of my bonsai outside is not frozen. Slushy at best. So this year I would wait until the ideal repotting season to do any machine carving, especially with equipment like die grinders or mini-chain saws. Usually a Dremel type tool does not cause excessive vibration. The more gentle your tool work, the less need for being careful about timing.