Parallax only effects a shot if your eye position is not centered in the scope. There is no direct answer to the question because it depends on how far off the center of the view you are. In most cases you would never experience maximum parallax error because you more than likely wont be at the extreme out edge of the view shooting.
The biggest problem with a scope thats set parallax free at a closer distance is how blury it will be at longer distances. For example, I set the parallax on a scope for a friend who's shooting crossbow at 30yds and now the view is extremely blurry past 100yds...
If the scope is set parallax free at 75yds, its more than likely designed for intermediate closer range shots. If your going to do alot of shooting at 300yds, I would go with a scope set parallax free at 100yds, that way you have the best of both worlds for both close and longer range shooting. Obviously your not trying to shoot one hole groups or you wouldnt be using a 2-7.... I personally favor a 2-7 for 50-300yd big game hunting...
MO