All I can say is: what do you mean for bumping?
A small amount of contact is acceptable in NASCAR and touring car championships, due to the closed wheels and the presence of bumpers. In narrow tracks, where clean overtaking is almost impossible, slower drivers use all defensive techniques allowed: some are considered rude. If they are considerable slower, all, marshalls included, allow a small amount of 'bumping', i.e. hitting the rear of the car in front of you WITHOUT sending him out of track, or crashing in its rear on braking point. In most cases (narrow tracks, too much little difference in lap times, lack of top speed) you want to avoid large contacts, and without some collaboration of the driver in front of you, overtakes can be impossible. With bumping, you remind this guys that you are faster, and discourage them using ALL defensive techniques. Next step is trying a real overtake. If he's wise, he lets you come in without narrowing the line, braking on apex, driving you out of track exiting a corner and such rude techniques.
Bumping is not acceptable as a overtaking technique.