Never moved under a wintec 2000, treated myself to a lovely Ideal Event saddle and the polypad skidded out from underneath it every time.... sold it in the end!!!
I love polypads - esp for training, and have never had a problem with them. I think you can get shaped ones nowadays anyway, which will stop any possible slipping.
I have a classic which used to slip back but when i pulled it forward a tad it never moved. I have recently baught a performer (cost me a bl**dy fortune!) which is shaped round the front and it never moves its fantastic! Also it has a nicer lining which helps keep the horse cooler so you are planning on buying a poly pad I would say a performer poly pad is a good investment!