Indeed I do not. I spend all I have to eat twice a month at a restaurant, which comes to a whopping twenty dollars in total. If I were to tip, I would only be able to eat one full meal and half of one the next time. As someone who doesn't eat very well or very often, I'd rather tip 0% and be slightly more fed than tip 15% and be less fed.
You should probably spend those $20 on buying some better food at a supermarket if you're that hard up for cash.
For $20 you can get two weeks worth of Calories from rice.