Do you really have to trim their bums? I plan for this spring is two fold. One to get a laying flock of colored eggs: Marans, Ameracauana and Olive Eggers. Second is to breed rainbow Orpingtons. LOL. I love the Orpingtons so much! Such beautiful fluffy, sweet birds. They are good layers, but can also be used for meat. Right now I have two Buff Orp pullets and three Lav Orp (English) eggs in the bator. So far all three are doing well. I have an order in for BBS Orp eggs and am trying to get some Jubilee eggs. I only inted to have Orp roos, but do not intend to separate the colors for breeding. Thus, my rainbow Orpington flock. I just want to have fun with the colors. No intention of breeding for a show flock, just good layers with lots of color.
I like that, a flock of "good layers with lots of color."
Bums have to be trimmed on fluffy birds like Cochins (IDK about Orpingtons) b/c it's hard for a roo to hit the mark with all that fluff and feather in the way. Trimming the bum feathers gives a greater fertile rate and therefore a better hatch rate. I understand that Cochin fertile rate, for instance, is about 50-60%.
I'm giving Cochin as an example breed b/c I know more about them than Orpingtons, but I'm pretty sure that Orpington is similar to Cochin in the fluffy department.
lol
No, seriously though, while I have never had to trim a chicken bum, if you were to just trim around the vent, you would be safe. But I am sure Piglett or one of the other more experienced BYCr's could answer that for you. 
