There is a certain school of thought which advocates silent rolls. I believe Donovan Waite is a proponent of this. It's worth considering, if only as a change in mindset.
Personally, I often slap on the mat but whenever I've gone into a forward roll off the mat I've never slapped. This includes two forward rolls off my bike, plus numerous times on concrete and dirt/grass. I would suggest that it's not mandatory but it sure does help to dissipate the energy of a throw.