Remember, when you're upvoting or downvoting a post, you are primarily concerned with the worth of that particular post. Whether your upvote translates into XP or not is a matter of chance.

There's no way to separate our feelings about the poster from our feelings about the post. Even so, votes should be primarily about the post, and at most secondarily about the poster.

So my answer would be: there's no such thing as too much XP. I upvote merlyn's posts whenever he makes a particularly good point, just as I'd upvote any new user whenever she or he makes a good point.

stephen