You mean the tail must be square, ARFs have the incidence pre built in 99.9% of the time. Yes, the elevator must be square on both sides to the wing in oder to fly straight on turns/banks. Rudder also perpendicular to the stab.

Hinges are also overlooked. most common are CA hinges, When used, be sure to use CA on both top and underside of hinge, or they may pull out. A good Pre Flight test should also be performed, Pull on the hinges, make sure all Servo horn screws are tight... Etc...

And also: NEVER use quick links on the elevator, Unless you square the rod on the contact area, (Dremel), and you use Thread locker on the set screw. I've seen too many planes be lost due to cheezy quick links on the elevator!

There are more, but I'll leave them for someone else