I always tip, unless the service is extremely poor. There were a few times when I didn't leave a tip at all, but it was justifiable. I feel good when I leave a tip and always try to leave an amount that makes sense. I once had a food delivery person actually demand a tip, rudely. I let the silence grow, then asked pointedly, "How much do you want?" She looked embarrassed and mumbled something. I tipped her anyway.
I used to work in a job where I relied on tips. My mother was very ill and I was the sole breadwinner for her and three younger siblings. Those tips really made a difference! I think that it is plain mean spirited not to leave a tip.