I have worked for and run several shops. I don't know a one that won't, as you say, "service" a tank from any customer that walks in. Maybe things have changed? Any shop that wouldn't is awfully short sighted, what with people moving from one area to another, receiving presents from someone else... inheriting gear from others... "Servicing" that gear is a great way to make new happy customers and a great way to boost profits! Inventory for providing that service is very cheap! (a lot cheaper than carrying an inventory of cylinders that often end up sitting around and being clearanced at closeout prices... or used as a "loss leader"). Cylinders are not usually a "high profit margin" item for dive shops... but are carried themselves as a service for customers.
JMHO... based on common sense and experience.
And yes, Dennis has always been A1 to me! I am all for doing business with dive shops whenever possible. It's darned hard to keep them around when people are buying their gear mail order or off the internet... or from general purpose sports stores that have no diving professionals on staff.
The purpose of my previous post was not to promote doing business elsewhere than the LDS, but to make sure something wasn't misrepresented.
Find a LDS that will readily and happily service gear purchased from other sources, and you will find a shop with a great attitude that deserves your future business! Buy from them and support them, so they are there when you need service later. Your local fire station may have a cheaper connection for hydro testing, but they can't help you with other dive equipment needs.