There is no "one size fits all" rule. Many shops, including me, will not work on a brand for which they have no manufacturer specific training, manuals, parts, authorization, and resource/contacts for questions that arise. If you bring me a brand I don't service, I will get it to someone who does, and is trained, authorized, etc.
Now if an online brand is willing to freely provide me the same support and authorization for a brand I don't sell, I would have no problem with that.
But many will NOT just plan to figure it out as they go no matter what the item.