I've also heard of people using RainX on the shower doors. When we had our new door installed I asked the installer about both - he said to not do it. Both put a "waxy" type of surface on the glass. It may make the shower floor slick, and he just didn't see a huge benefit to it. He felt a quick squiggie on the door and you'd be fine. Both of those products then make the door more difficult to clean. I choose to just wipe the door down after each shower and once a week go over it with a windex type of product.

The manufacturer of my fiberglass tub (can't recall which company off hand) recommends paste wax every 6 months to protect the finish. I've never tried it on a shower door though.

A shower door is made out of glass. You can wax it. Wax will not ruin the door. If you polish it appropriately, there will not be any waxy film. If you have ever waxed your own car, you know that if you rub it out properly, there will be no haze, only a shiny finish that repels water. I have used wax on my shower and like how it works. The wax will not run off the door and onto the shower floor. That being said, you'll probably still have to squeegee it or wipe it down with a towel after showering if you want absolutely no spots. But there will be a lot less water to move around because most will simply run off.

I used Rain-X and it is working out well after 3 months. There is not a lot of water splashing on the glass, but what does get on the glass beads and wipes off easily. I am not seeing any residue problems when the glass dries. There are no clouding or cleaning problems. I plan to continue using Rain-X.