I've found that the POI can vary enough to notice when a rifle is held differently. For example, between resting it on a standard benchrest as against holding it in both hands, with the back of the weak hand rested and the butt against my shoulder. When I'm sighting in for hunting I use the latter, so that the POI will match up to what I'll get with an improvised rest in the field. It will also align near enough to prone unsupported or sitting. Of course for competition shooting the zeroing is done from the shooting position, and refined with the sighters before shots to count.
Sometimes the zeroing is done from a bench, holding the rifle as described. Sometimes from a rest over a vehicle bonnet ("hood") or prone behind a backpack. With a rifle which boots a bit I prefer to be standing or kneeling behind the rest, rather than sitting on a chair hunched over it.
I've never had any use for Lead Sleds, both because of concern about what they'll do to the rifle and for the reason that the POI will not match what I'll get in the field, which makes the exercise a bit pointless.
