Never, ever ask.
What are most people going to think or say upon answering the call or servicing you at the front desk? They might not even know they can ask you to leave, but by your asking, one could infer that they now have that option. They aren't going to be the one to start a "Starbucks thing" at their work. They are probably going to be thinking about liability, that they themselves might be sued. They will usually invent some policy or law on the spot. They might even make a diagnosis of your mental state for you at no cost. Any of these, or a hundred other thoughts will come to mind. Even gun stores have been known to ban open carry.
What they are not thinking, is helping you with your open carry activism.