Actually, when I judge, I start sorting the class as the class is entering the ring. Regardless of whether it's a regular class or a showmanship class, it's important to realize the class starts when you walk into the ring, and you need to be competing from that moment until you walk out with the judge talking his reasons on the mic. Don't wait for the judge to start pulling or placing entries to get serious, and don't stop showing once your calf is pulled into line.
As far as the specifics of the class lining up & the judge's way of doing things, it varies from judge to judge, and sometimes even from ring to ring. When I get to a show, I study the ring...notice where the light is, where the entry & exit gates are located and where the spectators will be, and I try to develop a plan that makes sense for both me and the exhibitors & allows people watching the show to see what's going on. Every judge may do things a little differently, but once he or she starts, each class should follow the same pattern. The best thing to do if you can is give yourself time to watcxhma class or two before your class, and that way you'll know what to expect. Some judges place from the bottom up...some start with 1st place and work down. For me, that depends on the size of the show. More entries mean I work from the bottom up so I spend more time evaluating the class winner than I do the bottom end cattle.
Particularly in showmanship classes, a judge may pull cattle or line up exhibitors in no particular order, and that's when it's very important not to act like the class is over before it's actually over. Just because you've been pulled doesn't mean the judge's mind is made up!
The more classes you can watch and the more shows you can participate in, the better. Things will start to make more sense as you go along.