I'm lucky enough to have a relatively large choice of suppliers within my working range:
Yale - 2 locations
Colonial - 2 locations
Billows - 1 location
Rumsey - 1 location
United - 1 location
Yesco - 1 location
CES - 1 location
Out of that list there are three that I use way more than the others. They have more knowledgeable staff, better inventory and comparable prices. Inventory is starting to suffer as more suppliers move to a "Keep two in stock" approach and more and more things are second day items. That may affect who I do most of my business with since it can really get ridiculous at times (ex. not having enough Maestro dimmers on stock to fit out one kitchen...). The staff at one of the suppliers is SO BAD that I really only use them in an emergency. I've literally walked in on a sleeping counter guy who never woke up as I walked around looking for what I needed, didn't find it and walked out. Geez. That one specializes in supplying bigger contractors who do housing and commercial spaces - so not much business after 8 AM
The profile of my work doesn't lend itself to me investing in a pile of gear sitting on shelves collecting dust. There are some things I "stock" but I keep inventory low, buy what I need and use stocked items to fill in when I'm short on something. I've seen companies that keep a large stock in their shop go belly up for different reasons, and the only advantage then is to the vultures who can come in a get good materials for pennies on the five dollar bill.
It's rare that I have to leave a jobsite for materials (as it should be) but I don't agree with never going to the supply house. There are some real traps I've seen other contractors fall into with that approach. For example, the owner and his guys get behind on new materials, tools etc. because they never see anything other than what gets ordered. If they also don't read trade publications, it's easy to get 5-10 years behind fast. Another issue for resi contractors can be awareness of pricing among the employees. I've seen employees recommend high dollar ET8000 series timeclocks when a simple T101 would have been better suited not realizing that the astronomic version costs almost $200 vs. $50 for the mechanical version. Not the best way to make your customers happy...