Re: slap or silent in ukemi?
do Ukemi - let your body adjust to the specific situation and act in accordance to it.
Slapping is very efficient and even neccessery in some fast and hard falls, in such cases, it can be done on hard surfaces too (concrete, asphalt, etc. been there, done that), a broken hand is better then a broken head/neck.
Yet, slapping is not a must have, it is a chiocc. I would not slap if the fall is does not require it (speed, angle,surface, ...). And I would be even less likly to slap during a roll.
Practice both and let your body make the choices for you.