In some positions in doesn't matter too much i.e if you play a CMF in DMF whilst others it will matter. I had Odonker and bought him to play RMF but he kept going into the centre leaving me open on the right. I am having the same sort of problem with Maxi Rodrigez.
Best thing to do is experiment with the players you have and if you want to play some out of posistion just see how they react during the match.