My answer is that it depends on the scope and application, but as others above have said this push to have the scope at "optical center" when zeroed really seems to be a recommendation only seen on air rifle forums. Go to powder burner forums, especially where longer ranges are frequently shot, and the question is "what cant base do I need for maximum elevation travel out of my scope?" On the powder burner forums you almost never read anything about finding optical center on the scope-- unless someone is wanting to reset their scope to center prior to zeroing it to see how much windage or elevation is needed when they zero the scope if they think they're chasing a scope base or ring issue.
Many cheap scopes don't track very well if you dial for elevation or suffer from very obvious degradation in image quality when not near the optical center... and in that case it's best to use holdovers only and adjust the cant in the mount/rings to have the scope zeroed close to the optical center. I don't mean to be blunt here, but many people put rather inexpensive scopes on their air rifles that often suffer from these kinds of tracking and/or image clarity issues-- and my hunch is this is why we see recommendations on air rifle forums for zeroing the scope close to the "optical center" as the inexpensive scopes many people use on their air rifles work and look best near optical center.
On more expensive scopes that track accurately and repeatably regardless of how far up or down the elevation is dialed, they may still experience optical clarity / image quality or vignetting issues when the elevation erector is dialed very near the upper or lower extremes of travel. For instance, the S&B 5-25s I have on my powder burners will exhibit minor vignetting when dialed within about 5.5 mils of the upper or lower extremes of elevation travel. The Athlon Cronus BTR I have on my Impact M3 will start to lose a little bit image clarity when dialed within about 7 mils of the upper or lower extremes of the elevation travel. I also notice very slight image clarity degradation when dialed within about 5 mils of the elevation extremes on my Premier 5-25s & 3-15s, Minox 5-25s, and Tangent Theta 5-25s. None of these scopes have tracking issues or zero retention issues when dialed near the upper or lower elevation extremes; it's just minor image quality issues that appear, none of which would prevent you from taking a shot... but if you stare through them long enough in those "compromised" ranges of elevation travel it's a bit annoying to the eye.
Because of the image quality issues when dialed near the elevation extremes as described above I try to choose bases that will have the scopes zeroed just above those "image compromised" ranges so I get maximum available elevation travel with no optical issues at the zero distance / elevation setting (i.e. a base that leaves the S&B 5-25s a minimum of 6.5 mils up from the bottom of the elevation travel at the zero range, the Cronus about 7.5 mils up from the bottom, and the Premiers/Minox/Tangents about 6 mils up from the bottom.) Doing so provides maximum elevation up travel from your zero for dialing for longer shots but with no image quality issues at your zero setting. You can of course put more cant in the base for additional elevation travel-- but then you have to deal with compromised image quality when shooting at closer ranges at that zero setting.
Having your scope zereod near the bottom of the elevation range is a bit harder on the erector spring as it’s compressed more-- but that should not matter to a quality scope. If you're worried about excessive compression on the erector spring weakening it over time just dial the elevation turret up when you're not going to be using the rifle for a long time, and dial it back down to the zero setting when you go to use it. Just remember to dial back down to zero before your first shot goes way over the target and you're scratching your head where it might have hit...
Setting Scope to its Optical Center —OR— to Max. Elevation Adjustment Range?
The posters on airgun forums seem to be very concerned to keep the scope at its optical center.
Not sure what to make of this stark difference between the two shooting communities..... 
