I'd be confused/amused if I walked up to a line that Kevin Standlee was sitting at and he said "May I help you?" I would probably expect something more like "Hey Twi" with whatever the standard patter for that station was attached.

I think I'd probably use, "What can I do for you?" if I was in a "service" position (say, Registration or Program Ops Reception or Newsletter or Info Desk).

Or more likely "Can I sell you something today?" if I were behind a bid or convention sales desk. :)