At most shows they will do as Louie experienced. If all of your FPs use the same feeder and nib you can have one out that can be switched among your pens. It is a pain in the rear but can work as well. It that manor you will always use the same feed and nib for the shows never having to throw one away because it is used.
I do as Mike and Linda but with a twist, I keep 1 feed and nib complete with the housing loaed with a Private Reserve cartridge in medium and one in fine for the Jr. series and the samr for the larger pens. Then I will put the housing on the pen that they like so they get a feel of their NEW pen. Also use a good grade of paper such as Rhoda or Clairefontaine.