I agree that one should certainly try to keep within +/- 100 feet. However, I also agree that it's not always easy. Today there was no significant turbulence, but flying at around 3000 feet below broken cloud at around 6000 feet, I had to watch the altitude all the time. I'd get it perfectly trimmed at normal power and speed and it would be alright for 30 seconds, then the altitude would start to drift up or down. I'd probably flown into slightly warmer or cooler air, or into a slight up-draft or down-draft, and now it was out of trim. I had to keep re-trimming. And that wasn't in any perceptible turbulence.
So to say:
Usually when the aircraft is hunting in vertical speed, that is a sign that the aircraft is incorrectly trimmed.
is true, but it may have been correctly trimmed a few seconds ago!