I don't think it's reasonable that one should rate everything. How much you should? Umm... More than zero?
I don't think that's something anyone else can answer for you. I would have felt like I'd have paid my due after 10, but I ended up rating all of them. But I've got loads of time on my hands from being laid off work last week.
Personally, I think there is some ethical obligation to rate some, but the number is up to you.