When one shops for 6-year-olds, should we let them choose their clothes?

Yes, I think its a good way to introduce children to the idea of independence as well as basic aesthetics, such as what goes well with what. Of course I don't mean that you give complete control to the child. You can tell the child what items to look for then let them choose a few, while taking the final decision yourself. It will help to build self-confidence as well, and also teach your child to concentrate and look properly. A fun way to teach :)