Not a server but as a customer I always prefer you take it while I'm still there, mainly because I tip in cash, tip well, and don't trust other customers lol.

My wife laughs, but if we're at a restaurant that doesn't have the little book thing, I'll leave the tip at the farthest end of the table/booth, slightly under a salt shaker or empty glass so it's visible to the server but not so easy to grab that someone can come along and swipe it. Over the top maybe but you all work so hard for your tips, I'm not taking any chances with you not getting it.

So if you ever find yourself annoyed having to reach for that tip under the salt shaker, just know that the person had a reason. A paranoid one maybe, but a reason nonetheless.