I think often the answer is "you won't know until you try it". Forward-phase, reverse-phase, symmetrical... Low voltage in particular makes things more complicated.
Why can't the LED controller circuits come with a calibration feature that detects (with user help) the minimum level the dimmer will support, and makes that the all-black point?
Partly because the usual minimum level the dimmer will support is off. That is the all black point :)
I know what you mean though. It'd be nice if there was some way to adjust them to change the brightness curve to match what you'd expect. It's one reason I'm considering something like the Phillips Hue bulbs instead of dimmers in the future despite the cost difference.