IMO social skills don't need changing. It makes sense for classes who have it as class skills to have better scores than others, but i know DMs would take it into account if your fighter had maxed out the skill and taken feats to boost it.
Petty much this.
Every DM I've encountered takes this into account - if there is a fighter who has taken lots of bluff, I'm gonna reduce the DC a bit, if they have taken Skill focus, I'll drop it heavily, if they have drank a charisma potion, have skill focus, invested in the skill and have an item with the skill? They're probly just gonna win on a roll of anything but 1, tbh.
That's me, and every DM is a little different, but every DM I've seen takes investment into account.