If a sandwich shop has a "tip" jar by the register, the owner is stealing from his employees. In the US if the employee can get tips then the owner does not have to pay minimum wages. When I go to a subway shop, I stand and go through the line, and take the food to my table, get my drink, take my trash to the trashcan, I don't expect to pay a tip, so I don't. If they should put a tip jar by the register, I am going to start giving them hell. If I go to a all you can eat buffet, I will normally give a tip, if they bring something to the table, and I see that they also clean the tables.
Most people leave the tip on the table, but I have learned something from my wife who is much older than me. Being that we are disabled we give ether one dollar for each person at the table or ten percent of the bill which ever is larger. Anyway when we are ready to leave my wife will call the waitress over, and personally give her the tip and thank her for giving us such good service. She does this even if we didn't get that good of a service. But I have noticed it don't take long for them to start remembering her and giving excellence service. I do believe the waitresses like that personal thank you even more that tip.
True story, about 20 years ago me and the wife were on the way to las vegas, we stopped at this truck stop to get coffee. The counter was full so we got a table. We got our coffee, but our waitress went in the back and did not come back out to give a refill. I finally got up with the two cups and headed to the counter, where I was properly served by the other waitress. My waitress finally came with my bill for the two cups of coffee. At which point I put a penny on the table and gave the other waitress a five dollar bill while the first waitress saw it. The next time I saw my mom again I told her this story, expecting her to agree with me, (She was a old time waitress). Instead she told me that I had made a mistake, she said you should not have left the penny. It seems that in the depression, leaving a penny was code for (you have given me excellent service but I cannot pay you for it at this time.) So if you have bad service don't leave a penny!