The Oxford English Dictionary defines a tip as “a small present of money given to an inferior”. It's an archaic and embarrassing custom that should have faded out decades ago.
When you sell your services, both you and your customers should know the price explicitly in advance: if you have a price list, it should be printed there. If you give a better or worse service than they expected, they have a number of options. They can thank you or avoid thanking you; they can use your services in future or avoid using them; they can give their opinion of you to your employer and to their friends and colleagues. These options should be enough: tips are not necessary.
As a customer, I really hate not knowing how much I'm supposed to pay someone for a service. It's no good saying, "Whatever you think is appropriate." I may have no idea what is "appropriate", and I don't want to waste time worrying about it. I just want to know the price, and I want to know it in advance, not after I've already accepted the service.
The customary level of tipping varies wildly from place to place. In my experience, a German waiter expects small change; an English waiter expects about 10%; a Spanish waiter expects something between the two.
In a Miami airport snack bar, we emptied out our change for the waitress, which came to a tip of about 10%. She didn't accept that, and demanded 15%.
A tip is, by definition, a present. To solicit a tip is rude; to demand a bigger tip is shockingly rude. If a "tip" is obligatory, it's not a tip at all, and it should be printed on the price list with any other obligatory charges.
Also in Miami, the baggage handler who received our suitcases for transfer to the ship demanded at least $1 per bag. He was polite but firm. We gave him the money, not wanting to endanger our suitcases for the sake of $2, but we were shocked again. We didn't even receive any significant service from him: he just took our suitcases.
On the ship, the tipping wasn't cheap but was at least formalized. We were officially invited to tip the waiter ($3.50 per person per day), the assistant waiter ($2.50), the head waiter ($2 to $3.50), and the cabin attendant ($3.50) — all of these to be paid in sealed envelopes at the end of the cruise. We tipped the recommended amounts, except that we gave a bit more to the young Latvian assistant waiter, whom we particularly liked. We hardly saw the head waiter, so we gave him the minimum. I thought about giving less than recommended to our cabin attendant, who was rather inefficient; but she was friendly enough to us and we decided not to be unfriendly to her.
Our waiters were pleasant and very attentive, but both of them were obviously trying to be “characters” in the hope of attracting a good tip. I prefer waiters to be unobtrusive, and I'd prefer a cruise with a no-tipping rule.
Some of our excursion guides politely hinted at the possibility of tipping, but we didn't tip any of them.