Manufacturer data on stuff like Rain-X varies by manufacturer. HJC recommends it. See Shield Care. I've heard some other manufacturer say you will die if you say Rain-X while wearing their helmet, so YMMV.

I happen to wear an HJC Helmet so I don't worry too much about it. Even if I didn't, the worst that's going to happen is that over time the shield could become more brittle and discolor. With the amount that I ride, I find I need to replace my shield about yearly anyway, so it doesn't have time to discolor too much.

 

Sweet, cuz I've got a scorpion helmet so it has the anti fog crap on the inside but that doesnt really help he outside in the rain/fog. And yeah I carry my helmet around all day at school so within the first 2 months of having it i had fairly noticble a scratch on the righ side of the lens. Guess it wouldn't matter if I rainx'd it since I'll probably end up with a new one every ear and a half haha

 

So far what I've learned is:

1. Turn your head for a second when it is clear ahead of you to get rain off your shield
2. Painted road is the devil. I've made it my mission that even when dry out, to be in the habit of stopping where my feet won't touch paint so that I don't accidentally do it when it's wet. First time riding in the rain, I pulled up to a stop sign, put my feet right on the big white line for the cross walk, started slipping on both feet and luckily I was able to let out the clutch and give throttle and didn't fall.
3. If you have a weather liner, pack it or wear it. If you're not sure whether you need it, BRING IT. I decided not to today and it rained, and my jacket got me soaked, then freezing cold from the wind.
4. Pay close attention to drivers, more so than usual. Especially in my area, it doesn't rain frequently so people tend to drive very strangely in the rain, reducing speed, braking frequently, reduced visibility, etc. etc.