It is generally disrespectful to go into another school as a guest and directly ask the instructor to roll. Not everyone finds it disrespectful, but I would say the majority of guys in BJJ would take that as disrespect. When you ask someone to roll so that you can see how your skills measure up, it definitely has the undercurrent of a challenge.
It's one thing to ask a regular training partner to roll. But when you are a guest in a new place and it's clear that you are trying to measure yourself against the other students as well as the instructor, it definitely takes on the feel of a challenge.
It's just important to realize this because the truth is that a lot of instructors (the Brazilians especially) will take that as disrespect and retaliate against you during the roll. You might be less likely to get that reaction from an American, but you are still taking a chance.
When you are a guest in a new place, it is better to follow the "don't ask anyone else to roll" rule until you get a feel for the place. Other guys will approach you if it's appropriate. You especially do not want to be asking the instructor to roll. If he wants to roll with you, he will ask you himself.
I'm not even making a judgment on whether or not this is the way it should be. It is just the way it is. There are a decent amount of schools out there that do not take well to being "measured" by visitors as you say. If you give off that vibe, they will do what they can to smack you down with little regard for your safety or well being.
The point is that you need to keep a low profile at first when you visit a new place or else you run a pretty substantial risk of having guys gunning to take you out.