My view is that a 'tip' is always earned !
If the taxi driver helps with bags ( or advice). if the waitress/waiter made the evening more enjoyable,if the barber/hairdresser made the ordeal less painful, etc i would add something extra.
Often wander down to a 'carvery' where it is self service, which I perfer, so NO tip.
But I might be biased, as have never worked in the service industry.