It's a little awkward if you're someone who doesn't like the server seeing their tip (for whatever reason) while you're still there and interacting with them. They have to run the final transaction, including tip, while you're still present, if you're charging it to your room.
We had a couple servers during our recent trip who, in my opinion, earned nowhere near 20% (others earned more). But, because I don't like conflict, I tipped them all at least 18-20% since there would still be interaction when they brought our final check to sign.
You can always tip in cash though, which is what I'll do next time.