Truing stands are unreliable for checking dish because they're subject to various errors.

Depending oh how they support the axle, they're vulnerable to errors introduced by a bent axle, or wear or distortion where the axle meets the stand. Often you can see these errors by positioning the wheel but not tightening it in place, then using a cone wrench to rotate the axle. Errors of the type I described will move the rim side to side, throwing the dish readings off.

Then, depending on the construction of the stand, there's always the risk of the centering calibration going off. The Park TS-2 stands are notorious for this problem.

However, there's no need to buy a dishing gauge, since a very accurate one can be made at zero cost, whenever needed, using a table, 3 soup cans, and a stack of coins.