I remember Donovan Waite from his time in England - silent rolls tend to be popular in the UK (and I've never been told specifically to slap/tap the mat unless it was absolutely necessary).
We practise silent rolls regularly as it is a good way of showing that the movement is round and no lumps (ankles, hips, shoulders) are making rapid contact with the ground.
However, this is totally different from slapping the mat to reduce the kinetic energy (which is used when you can't convert that effectively into a force which will make you ukemi smoothly out of it).
As with Colleen, I never had a problem using a slap when they are needed (I think if breakfalls are practised, the in-between bit of breakfalls and ukemi come naturally).
I think this illustrates the problem of passing on an exercise in aikido - if you don't know why you are doing them they can actually make your technique worse - therefore; as the 2nd post said; ask your sensei.
P.S. I don't think slapping the mat whilst doing reverse ukemi is obsolete as if you are thrown very hard backwards it can be very useful!