I use Leupolds and sight in at 9 power, then check at 6 powder or so, never noticed a difference...
One other thing I do when sighting in is I clean between each shot...Usually 2-3 moistened alcohol patches, both sides...Then a dry patch, then a lightliy oiled patch and another dry patch...This is how the barrel will be with my first shot so I want to duplicate those conditions...
In the field I carry a ziplock bag with patches in it...I put some alcohol in a Visine container and clean with 2 patches, a dry patch, an oiled patch and another dry one between shots in my inline...Once you get the system down you can be reloaded in 2 minutes...