I use my keyser all the way up to C and never have to retune. My flatpicking guitar's action is set at the lowest pre-buzz height, and the soft pad and proper placement of the keyser ensure that it stays in tune all night.

If your capo puts your guitar out of tune, then your fingers do, too, since they are performing basically the same function. Experiment with new capos, better capo placements, and varied action heights...if that doesn't work, get a new guitar.