I prefer RAT view and the color scheme plus my votes and timestamps give me a good indication what I already saw.

(Obviously you've already read where you voted. Well at least you should have ;)

If that's not enough, I'm pretty sure you can adjust your CSS settings to mark or hide already voted posts.¹

Edit

I've also a Nodelet hack in place which pre folds all threads, hence I can ignore them easily by title, even if they have a lot of traffic.

footnotes

¹) see RAT Style Settings