I started to write a big long post about all this but I'll boil it down to a couple of answers-
It really all depends on what kind of precision that you are looking for. A guy trying to hit a prairie dog at 250 yards will need to pay a lot more attention to the details compared to someone that is running M193 thru a Colt 6920 with a red dot, engaging steel silhouettes at half that distance.
For me, and for the sake of simplicity, I zero my red dots at 25 yards and magnified optics at 100. The 25 yard red dots will be used for defensive/close scenarios while the scoped rifles will be used anywhere from 100 out to 1000 yards and beyond.
After zeroing, I use a ballistics calculator to figure out the holds.