To me a good professional approach that's been fairly charged, deserves extra as a token of appreciation, even if it is a bottle of very good whatever they like to drink, and occasionally if I can, taking them out for a meal.
There are way too many unprofessional 'professionals' out there, with supposedly relevant bits of paper, that have a very bad approach to what they assume to make a living at.
The ones that do it right don't tend to get the recognition or encouragement they deserve, and it is that little extra something, that tells them I noticed, and if it helps to keep them encouraged to keep doing a good job, well, it's the least I can do.
I have a couple of very good gardeners (stuff I used to like doing) that charge £10 an hour each. They tackle it professionally, don't hang about, and do a good job. With a recent big project
they asked me how I wanted it to look, so I encouraged their creativity by telling them they have far better artistic sense than I have, and do something that looks right to them. It turned out great.
When they arrive I make them breakfast, at lunch I make them lunch. They like the food
I make. If it's hot I have bottles of spring water
in the fridge ready chilled for them, etc.
I supply any necessary gear
that they might need (e.g. a big good quality disc cutter
with a big diamond blade, for customising paving, and a wood chipper to turn waste into useful fuel), and make sure they have all the necessary high quality protection gear
(dust masks, eye protection, hearing protection, gloves).
They are good people, and if we have had a particularly good and productive day, I give them a bit extra. They are always keen to come back and put in another day.
At least one of them might even end up doing a bit of crewing
PS. I have tipped a Dr or two along the way. But I haven't had a good enough Dr to tip, for a very long time (in fact they should pay me for the mistakes
they have made, due to not paying attention, that have nearly killed me, and would have killed me if I hadn't been as fit as I was, at the start).
My father has just come out from an extended hospital stay, and I have 'tipped' the good people that looked after him, with some bottles of very nice home made liquers (with spirits made in my own still) for them to have at their staff Christmas
do. It's a personal touch with high quality things (e.g. a vanilla pod calvados) they won't be able to get anywhere else. When the plum brandy is ready, they will have a bottle of that as well. I also bought a load of their raffle tickets which will help their ward.