Hi John and Hank,
isopropyl alcolohol is a good grime remover and if you use it only once or twice a year your window should be fine.
But any alcohol is also a solvent and too much can make PVC brittle.
I have read somewhere that Meguiars and Griot have an additive in their polish that keeps PVC soft, I don't know what it is, and that could be marketing speak.