My new(er) mortice guage is by Joseph Marples, and the stock and stem are nicely square to one another. It had three minor problems, all very easily solved. First, the marking spurs were not quite the same length; two minutes with a fine file cured that. Second, the screw adjuster was very stiff. That was cured by carefully easing the spring washer holding the adjuster screw away from it's mating face with a small screwdriver - just a smidgeon, until the screw turns freely without endshake. Finally, it came fitted with a single marking point opposite the twin mortice points, on which I snagged my thumb a time or two; catch it in a pair of pliers and take it out. A tiny spot of oil on the screw, and the whole tool works as well as a vintage one, now.
Joseph Marples are not cheap, but I reckon they're worth it. Either that, or a good vintage ebony-and-brass one.