Or only if you press the roll. It's a core tenet of the skill rules that you only count ones if you press a roll anyway. Would be nice to not have exceptions.
Either way I'm also hesitant about the always prone rule as it currently stands.
Fair comment on the pressing..
On a related note, I'd suggest adding a caveat to the 3rd level Steady Feet talent to say the character can't be Shoved by someone of the same or smaller size. I imagine it's easier for a Giant to Shove you than it is another human or halfling.