I normally trim one of the nobbles off at each end (bottom), which is approx 20mm from memory.
You will notice if you buy coilovers etc, they come with much shorter bump stops. The standard ones are designed for standard suspension travel, so the danger is on lowered cars, you'll hit them more frequently than necessary, which can cause harshness.
It's not essential and don't do it if you're not comfortable with it, I'm just pointing out a potential issue, which is harder to rectify after the suspension is fitted. Having said that, you can trim the rears in-situ with a stanley knife, but not the fronts.