Interesting because I have seen seble say there is no marked difference.
ie... practice 10 minutes of Man and 10 minutes of Press and only play man-to-man, the improvement between the two will be similiar.
I know I've switched Off/Def numerous times and have never seen an IQ change for a set I played, but never practiced. Maybe 10-15 seasons worth. Some guys have played 60+ games and not practiced a set they played. If it does still have an effect, I'd guess it's less than an overall 15-20 minutes of practice time per season.
There was also the theory that the reason a player's IQ went from F to D- was because they played against that off/def in a number of games. Which came from an answer given during a developer chat.
But I've had players go from F to D- in fastbreak when I had gone several seasons and never played against a fastbreak.