Still Learning, sort of right.
You are correct that you trim for a speed but the power setting must be appropriate for that speed.
In other words, if you trim for 90 kts and have, say, 2200 RPM all will be well and you will fly at a constant altitude. When you put carb heat in, the revs will drop and if nothing is done (ie pull back slightly on the stick) you will descend. If you plan the approach part of a cross country correctly you can trim for S&L and just reduce power to descend to circuit height without re-trimming.