You didn't say what you were working on, but I do mostly airplanes (and some armor) finished in satin or flat, so no polishing is required. I do sand with fine grit paper in between coats but only if I find grainy or high spots after a job. Only time I polish a model is when I'm doing a car or a gloss-finish aircraft, and I do that after the last coat (I use The Treatment Model Wax). I've read of some modellers, though, who polish the bare plastic of their models if they're putting on NMF paint, since imperfections easily show through coats such as MM Metalizers. But normally, I don't think it's necessary to polish between coats. As Trev said, this may prevent the next coat of paint or clear from getting a good "grip" on the surface and thus become easily lifted if you mask over it. Hope this helps and good luck to your modelling.