We competed with over an inch of suspension travel and didn't have an issue. Leaving the bumper zone should only be a problem if the robot is designed to, say, tilt backwards, to the point if it were set on a flat floor the bumpers would be outside the bumper zone. Tilting during the match is alright as long as the robot will stand upright flat on the floor.
At least, that's the consensus my team came to after a very lengthy discussion. We ended up with "paradiagonal" bumpers, sloping upward from the bow to the stern as a result
