It's hardly a problem.
When I had knackard mounts mine didn't rattle at all, but the second I replaced with polybush bushes, it shakes crazy. That is because the bushes are potent and with the steering wheel at a certain angle, it goes bananas - I just got used to it.
I went for TRD bush's as the poly's have got a bad rep for vibration, but the vibration from the TRD's is still bad.
I would definitely say its a problem, as people comment on the shaking of the steering wheel.
I had the same vibration before the TRD's were fitted so i'm assuming its not mount related in some instances although it is fine after a certain rpm.
Yes, the vibrate will vary depeding on car.