I was always taught that when approaching a dock --- give the dock worker a line with a loop on it and ask it to be placed on a cleat - that all line tension should be controlled by my crew with my direction.
There were many years when I took the same commercial boat to the same dock twice a day to pick up/drop off daysail passengers - I usually only tipped when some real help was given or needed, the dock attendants knew I was very grateful when the inevitable, occasional snafu occurred.
I was often the last boat in for the day and sometimes gave the Dockmaster a six pack to share out when their shift was over.
When I singlehand a private boat I always ask for whatever help I need and tip graciously and appreciatively.