Once you have left the tip, the money is no longer your money. What the server does with the money is the server's business, not yours.

Different establishments have different means of dealing with pooled tips. When my husband was a bus boy, the servers gave him a cut of their tips at the end of the night, and similarly gave the cooks and others their cuts.

A pleasant restaurant experience is the sum of all the service personnel doing their jobs. You are seated promptly, without having to walk by a bunch of empty, un-bussed tables, finding your own table neat, sanitary and tidy. (Thanks, bus boys.) Then you are served by your waiter, the food is cooked by a chef and sometimes brought in by a food runner. The restroom is clean and tidy. All those people contribute to your pleasant evening meal, and there's nothing wrong with the tip being shared by all who contributed. Team effort, team compensation.

Remember, your tip is still 15-20%. You don't tip anything extra just because a waiter and a food runner are involved.