Remember when thinking of assigning a skill or position: you are sacrificing 3Q/4Q, so 60-80% of a STAR. SO you should only assign a skill or position to 18/19 yo fast trainers because they will be your worst players and need to be replaced if they are not fast trainers over the course of the season and next.
If you have tokens to waste, go ahead and knock yourself out assigning positions and skills.