To cut, or not to cut..?
 

I have recently purchased 3 beautiful Dendrobiums in flower, one of them only has 2 flower heads left on it and I thought I read that cutting the flower spikes off before the spike completly dies will encourage flowering next time.

Is this true and does the same go for all Sp.?

As far as I know, cutting off a flower spike will not encourage future flowering. It does allow the plant to spend the energy that would have been put into that spike on growth, which will allow it to mature a new growth faster, but most orchids either must mature new growth before they flower again or must have a seasonal trigger to flower.

An exception to this is Phals - when you cut the spike about halfway (between nodes) they will often form a secondary spike from the original one. This is normally done just after flowering is finished.