It may need to be done from the utility safety standpoint, but from a tree care standpoint, the timing matters little because trees should never be pruned ("topped") in this fashion. For one, it creates long-term branching problems, and may also reduce flowering potential. Plus, if it were repeated, the trunk ends become quite knobby over time, look bad (the knuckle-like appearance they develop, as you noted), and also perpetuate the poor branch attachment problem that arises as new growth tries to replace what was removed.

Honestly, it's probably better for the long term to just replace the plants so they have room to mature unencumbered away from power lines, or replant with compact-growing varieties that won't get too tall to interfere with the wires. Additionally, we've seen cases where crapemyrtles (and other tree species) stressed by severe pruning fall victim to infestations of ambrosia beetle, which can kill a tree.

Some arborists and landscaping companies with well-trained personnel may be able to fix this damage over time by doing further pruning to correct poor branching that will result from the cuts, but we don't have information about which those are (Extension doesn't collect this info), so you'd need to ask around if you wanted to try to remediate the damage that was done.

Miri