When does one unzip the window?

With the soft plastic window the proper way that is explained in the owners manual says to unzip it before lowering and lay it flat in the well. This avoids the window being folded. Of course I never have done that with any of my '66 Valiant converts.
If the top is getting stiff it maybe dry rotting. I bought a parts car with a top that looked beautiful after I cleaned it but it was all dry rotted & it was a bit more effort to lower it. So you might need to start planning for a new top.
The cars with glass or hard windows are a different story from a '65 Valiant.