Mirror Offside?
 

Do you (rather should you) mirror the other AR's offside call? Yes? No? Always? Sometimes?

When I first began officiating, I never mirrored offside. However, once I began working upper level USSF games (State Cup, etc.), I was asked to do it by some of my centers... particularly after they embarrassingly missed an offside flag from the other AR. I always mirror offside now. I'm not sure you'll find it listed in any rule/mechanics book, but it's a good habit to get into.

as a general rule, you mirror the other ar when the cr has not immediately acknowledged that ar's signal (missed it).

This is correct. You should not be putting up your flag right away.