The correct response to "Can I get a quick true?" would be "Of course. We charge..." $10 for wheel truing, or $60/hour, or whatever the shop's price is. The important thing is to be firm and matter-of-fact. A board with the customary rates goes a long way toward holding the line so the customer doesn't think he or she is being swindled.
I used to be a schoolteacher. As a teacher you learn extremely quickly to be firm and consistent, because you have 30 kids trying to negotiate with you at the same time, and all of them act like the customer who wanted a free truing. Believe me, this works.