My zero depends. I picked 100 because that is the default zero I use for any rifle short of a .22. For my military stuff, for ACOGs, it just works great. I tend to zero most optics at this range too. I may tweak the zero later at different distances until I get what I want.
I still zero backup sights at 25m/300m --the army way. Because I don't use them much, if ever, I just want a simple zero that works when I do. Not the best, but it works well for things like this.
For match shooting with irons, with the carry handle on an AR, I have a dope book and use the zero applicable at the range. This changes at not only the distance, but also depends on weather and ammo. It takes awhile to accumulate good data, and a lot of rounds, but once you have it, a quick spin on that dial and you are dead on it. No need for Kentucky windage, no need to guess elevation. Focus on sight picture.
For a laser, for an IR laser, I zeroed those at 300m exclusively. They work VERY well when used properly.
Still, the zero is only as good as the fundamentals you apply and the ability to repeat body positions accurately. I've seen guys get great 600m zeroes one day, then go back the next day, same weather, wind, time, etc., and group a full six to twelve inches over depending on the position.