Aside from the issue I mentioned above, with Rain-X potentially damaging the plastic lenses, there's a bigger issue I'd be concerned with: What's in Rain-X, and do you really want a vapor chamber of it enclosed over your eyes for an hour or so every day of the winter?

There's another anti-fog product called Cat Crap, which is actually designed for sunglasses and goggles. Although I find that a drop of Dawn dish soap rubbed around on both sides of the lenses of my shades, then rubbed clean with the edge of a t-shirt works just fine for a few days before needing re-application.

Edit: From the Rain-X website (which might be another reason to steer clear of Rain-X products)

"Rain-X was acquired by the Quaker State Company in November of 1997, and then in December of 1998 Quaker State merged with Pennzoil to form the Pennzoil-Quaker State Company. On October 1, 2002 the Pennzoil-Quaker State Company was acquired by Shell Oil Company. Today, Rain-X products are produced and marketed by SOPUS Products."