Actually, the tip jar doesn't bother me too much. I usually don't tip unless it's a full service dining. But occasionally it's The Holidays, or (rarely) a barista really did a great job welcoming me and even spending a minute to chat with me, so I consider giving a tip. That tip jar is there simply to remind me, but I don't feel pressured.
The iPad PAYMENT/TIP SCREEN, on the other hand... does annoy me, and I feel like I will get the Evil Eye from the cashier if I click on the screen button that says "NO TIP"
This is from all the restaurants that offer the iPad tipping screen, so it's not just Starbucks, so many fast-casual restaurants shove this iPad Pay Screen on you, pressuring you to click to give them some kind of tip. 15% or 20% or 25% are the typical suggestions on the tipping screen.
YOU WANT ME TO TIP $3 BECAUSE YOU HANDED ME A FREAKIN MUFFIN??!! ARE YOU NUTS?!! 
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Most (backward) states have a below poverty level wage for wait staff, and even at that, some establishments KEEP all the tips. Yeah, so your waitress/waiter would make around $1.70 or so an hour, without tips. Apparently the states thought that people WOULD tip, and the wait staff could make a bit more than the going minimum wage.
UNFORTUNATELY, tons of people don't tip. They have some asinine idea that those wait staff are being 'greedy', or 'wanting the good life', and sure that happens, but more often, waitresses get pinched, and prodded and lectured, and harassed. Waiters don't do much better. And delivery people get stiffed quite a bit too.
TIP PEOPLE. They DEPEND ON IT. They make less than the guy that cooked your food! Also ASK if the establishment takes their tips. I've paid with a credit card, and tipped in cash to hopefully help the tip go to the person that served me, and not some callous turd stiffing their employees.
I tip a minimum of 15% in sketchy situations, and usually a standard 20% for everyone else. I tip the housekeepers at the hotels I stay at overnight, if I'm staying the next night. I tip people that carry my bags, I tip people that get me a cab, I tip people that give me a drink at the bar. I sometimes get more towels, I get people wanting to carry my bags all the way to the room, and want to take them back to the lobby. I get better cabs, sometimes. I sometimes get a half shot more (or more).
If you treat people like, well, people, they often treat you well in return. But don't tip to get 'extras', tip to feel better about how you are helping them survive, helping them to see that not all their customers are tight jerks who never tip.
I have to laugh. I returned to a place one, and the bell staff recognized me. One guy actually followed me into the restaurant/bar, and told the head waiter that I was a 'good customer'. We got a window seat, and they comped our deserts. It pays to treat people like they are important. And it can make you feel better too.
(I always tip the barista, and make sure they see that I tipped. It doesn't hurt)
Oh, yeah, the 'tip jar' at one restaurant I worked at, was emptied at the end of the night, and the money was divided up amongst the wait staff, minus a cut for the management. A HUGE CUT! And people that slacked off the night got the same 'cut' as everyone else. They lost a lot of good staff because of that. Tip jar or not, I try to tip individual people, and they often keep it. If someone goes above and beyond, they need some acknowledgement that I appreciated their effort.
