This is one of those things in TE that can't be given rules or guidelines.
The skills required for each position are different. If the player has higher % in the skills that are unique to that position (amongst the positions that he can play) then he will usually play better at that position.
Its one of those things that you have to analyse your players and their abilities. Failing that, change them around a bit during the match and see how their ratings go...
For example, look at these two Desert Rats FC players:
The unique skills are for a DC, Heading and Strength; for a DL (or DR), Crossing and Speed.
Syvertsen is better in Strength and Speed, Catracchia better in Heading and Crossing. After some experimentation in play, Catracchia plays better at DL and Syvertsen at (left) DC, so that's where I usually play them.