The first time you fly the model you should make all of the big trim adjustments. After that you may make minor adjustments depending on a variety of factors like how picky you are, the temperature, density, throttle setting, and how full the fuel tank is. I have a couple of planes that I need to adjust everytime I fly. When I take off I usually end up needing a click or two of up to account for the full fuel tank. Then as the fuel burns off I end up putting in a little bit of down. It isn't real necessary, but I'm picky that way.

If you find you have to make a big trim change between flights then you need to land. Something is wrong and you need to find out what.