Currently you can look at a page and decide to vote (up or down) based on what you see. If that's been sitting on your screen for awhile it may have vastly changed. If the post (question or answer) you're trying to vote on has changed then it should bring back an error saying "Post has changed, please refresh" or simply refresh it automatically.
Occasionally I've seen downvotes (with comments) given for errors in posts that I'd corrected 15 minutes earlier.
Voting involves a server call so this would not be adding any server roundtrips.