Re: slap or silent in ukemi?
I'd say learn both but when you're in the dojo, do the one everyone else does.
When I first started I was taught a slapping breakfall. Later on I learnt a few different ways as well. Depends really.
And of course you don't slap if you're thrown on concrete but as most of us train on the mats, it's alright to slap too. I find that in big crowded seminars, slapping helps to show your presence in the vicinity. In case some unaware guy throws someone onto you or steps on you while you're down.