I was always taught to trim whenever I'm not flying a "transient manouvre" (e.g. turning, spiral dive
).
I normally set carb-heat, power & flaps mid-way down base leg, then trim . Once turned onto final, make whatever adjustments are needed to make the picture out the front look right, re-trimming if necessary. Then - with about a mile to run, the carb-heat comes off and I don't worry about the trim any more. There shouldn't be any serious amount of back or forward pressure necessary on the CC at that time... If there is - then go around!!