Retrimming in this situation should not be necessary, assuming the up- and downdrafts are temporary. Because once you exit the up- or downdrafts you'd otherwise have to trim again for a stable cruise.
Agreed; it's a matter of judgement as to what constitutes a temporary change.
But if you're out there in the blue wild yonder it is far less stressful, and more comfortable for your passengers, just to accept the occasional altitude excursion as long as it's not too severe. In the end the up- and downdrafts will probably cancel each other out anyway.
Again, I agree with that. However, even out of CAS if you're getting a Traffic Service then from memory I think they expect you to stay at the level you've told them (unless you've told them you're manoeuvring), so they know what to warn you/others about. At any rate, I try to stay within +/-100 feet.