If the manufacturer says do not exceed 5mm, then they probably have a good reason for it. It could mean the difference between rubbing or not rubbing. Or unpredictable handling. It means they have deemed 5mm is ok, but anything greater than that, you are ON YOUR OWN. You will have to conduct your own research, development and testing.

It could also be legal reasons. Transportation authorities set a limit on how much wheels can stick out because they pose a hazard. So you could be looking at fines for doing it too (and then having to buy new wheels)

Change the wheel off set. 
if you have any specific propose for doing that and you know the results, go with it. 

for example we change our wheel off set of off-road vehicles to change the shape of tires, for better traction and more aggressive looking. 

If not, stick with stock/ factory wheel specs for passenger vehicles.