I've never ever had a problem rocking up to a guest just to say hi. When I do go over I always ask 'permission' from the crew assistant or whoever before I actually open my gob. I'll just say smiling to the assistant, "I just wanted to say hello if thats ok." Its important to say this first as a way of pre-emptying the enounter so you dont get asked, "which would you like?" as you said. And I only ever do it when there is no queue and they are just sitting there...I think thats the key.
I dont agree with your 2nd point really. I could rock up to any of the guests in the supermarket and not ask for an auto and just say hello and give a compliment. No money is invloved there. When you meet guests at cons, you're paying for the auto, not the encounter.
I think you'll find the majority would disagree with you there! Trust me, if people were only paying for the autograph and not the encounter, they'd buy online and not even bother turning up to the conventions. Many people I've met have told me this, and some don't even collect autos either!