Hi Andrew.
The use of a clamp is purely optional if you think it improves the sonics then use one. I personally use one and I believe it must improve the reproduction. If only by dint that it firmly clamps the record to the platter.
I personally would not use the type of matt that is mentioned here. but a soft one which allows the record to bed down
and take up any slight discontinuities in flatness. Try the patter on it's own with a clamp first.