Andrew R Stewart 

A trueing stand is just an easier way to hold a wheel and indicate the rim's condition. Generally a stand is not meant to be a dishing tool too. How the axle sits in the stand's uprights shouldn't be any more than a concern for not coming adrift during the use and should be nothing about the indicating aspect. A somewhat skilled person can "work" the wheel during the truing in a manor that won't challenge the axle's seating in the stand. What I do like about a truing stand for a beginner is that it removes the distractions of the rest of the bike (brake pads, wheel fitting square in the frame) and that the stand's indicators are easier to see, WRT the rim's sides, than using brake pads or a hex wrench rubber banded onto the blade/stay.

Since it is so easy to make a dishing tool from common stuff (a counter big enough to lay a wheel flat, some coins or washers that will be stacked up to support the rim at 3 points as it lays flat and another stack located under the axle, flip the wheel and note that axle stack WRT the axle/lock nuts) I see little reason to trust the stand (or your frame) for dishing. Andy