When mounting a scope, it's desirable to have the reticle in the center of the focal plain, that's where the best resolution and clarity happens, toward the edges isn't as clear, if you center the adjustments, you're centering the reticle within their adjustment range, if you have to adjust it a lot to bore sight it, you know right away that something is out of alignment, it could be the base(s), mounting holes out of alignment, bad rings or the bore is off center. If it's off a lot, Burris Signature rings with offset inserts can take care of it .
Knowing you used a lot of windage or elevation adjustment just to get bore sighted will also tell you may not have much left to make adjustments zeroing at the range, something that would be nice to know before you're sitting in front of your target scratching your head when your turret adjustment runs out of travel!!
Count clicks if you want to, you can also use a V-block, the mirror method is so easy tho, best done before you mount the scope, it's easier to get the light angle correct.
I mount scopes at home on my workbench, I like to shoot at the range and not play with my scope counting clicks!!