There's only 3 tradeoffs

Intelligence decreases when Strength increases
Endurace decreases when Intelligence increases
Strength decreases with Spirit increases

Note that this does NOT apply to Dawn of Souls. If you're playing that one you can level anything up as much as you like, you'll only be weakened by what you aren't leveling up and what you have equipped.

Also, keeping mages in the back row is an iffy tactic. However, it IS a good thing to do for boss fights. Your more physical classes will take more hits (note that this doesn't apply for monsters that don't bother with physical attacks). I keep mages in the front, there's no penalty for increasing endurance, and they need it just as much as the physical fighters do. I only swap them to the back for REALLY bad fights.