Although it isn't something I would recommend a newbie to do right off. I belive it is necessary to learn to do it toward the end of your lessons. I always teach this right before they solo. In fact one of the tests to pass is to change thier trims in flight and make them trim it themselves. I belive it is important to learn as many planes will fly differently and you need to know how to change the trims without looking at the radio.

Learn to fly well first with your trainer and then learn to trim it.