I would use a cheap cover for the windscreen and possibly Rainex on the side windows only - but not on the windscreen.
I have found from experience that once Rainex is used on the windscreen and wears out that the wipers for ever leave a fine mist on each sweep which is difficult to see through. It is very difficult to remove worn Rainex from the windscreen. Probably the best product to do that is Barkeepers Friend but it can be hard work.
You can, of course, renew the Rainex every couple of months all year long but that also becomes a flog and expensive. Certainly snow slides off if you use Rainex - so it is good on the side windows but not, in my view, sensible on a windscreen although it is sold as such.
A windscreen cover is cheaper, quicker and safer IMHO.