I tip to reward good service, not to ensure it as some signs would lead you to believe. If a take-out place cashier is friendly, polite, and my order is correct, I tip. If our hotel room is clean, in good condition, and supplies are replenished, I tip. More if they go out of their way (towel sculptures, handwritten notes, responses to notes or requests I've left, etc.). At restaurants and pubs, I routinely tip 20%, unless a significant portion of the bill is an $80 bottle of wine. On the other hand, on the very few occasions when service has been glaringly awful, especially due to attitude or rudeness, I have left 10% or even nothing. When it's so bad I leave no tip at all, we generally never go back....if we did, we'd probably be identified as awful people who don't tip and receive even worse service, so there's no point.
I like to tip... it's a nice, concrete way of saying "Thank you!," and it's difficult for me not to. On the other hand, I hate to reward rude, nasty, incompetent people on the rare occasions I encounter them in the service industry. I'll happily tip someone who is incompetent but pleasant and trying hard.