Depends on a few things. In general they don't. But here are some exceptions:
Timing to the bore. Elite Iron times their direct thread cans to be TDC (their thread over brake models are timed if you time the brake). KAC times the can to the rifle with the QDC mount and has a pin that lines up the can TDC so it has to be timed regardless of whether it's the brake or flash hider. In fact, KAC's MAMS brake I don't think needs to be timed except to time the can. In general, if the can was designed this way, the change in POI should be vertical only and repeatable. For direct thread, Elite Iron will actually send you a timing nut you mark on your rifle and they'll thread the can to match that so as to be TDC when installed. So if your can times to the mount, you certainly want to time the mount regardless of what it is.
Note: like was mentioned above, if not designed to be installed TDC, you may have to "play" with it to find out where to time it so it'll give you that vertical POI change vs. an angular one. This is more important with some cans vs. others but more or less up to the user to do or not. If you do this with a flash hider know that the tines may be "crooked" or rather, you won't have a flat on the bottom or top. This is a trade off --time the can TDC to your mount, or time your mount so that you have a flat on the bottom or top depending on what the user wants, which can be cosmetic or not.
Timing for environmental reasons. If you have, say, a 3 or 4 prong or a flat bottom flash hider you may want to time it so the flat portion is on bottom so as not to disturb dust as much, or you may want to time it so the flat spot is on top so as to aid in the use of optics, especially at night and especially with night optics. A 4 prong can be timed to do both to a degree. As in the above, it won't aid in timing the can. But know many, if not most, flash hider mounts are intended to not be timed and timing the can isn't considered an issue by the mfg.
Function or looks. Some need to be timed because of the design, flash hider or brake, because the slots may be designed to be in a certain location to work as intended. Other people just time so they look better, so the tines aren't "crooked". In that case it's cosmetic and up to the user.
Use good shims. I prefer the ones AAC sells (I'm sure other sell 'em to but I have a lot and haven't had to look for 'em). The peel washers can be a pain to use and I don't feel they are as good as dedicated, sized shims. Don't overtorque the mount, that's important too.
FWIW, I don't time flash hiders unless they need to be, such as with the KAC mounts. With AAC or Saker flash hiders, I don't time 'em, I let 'em line up where they line up. If the mfg. indicates to line up the mount so the can is TDC, I will. If I have questions, I generally call the mfg. and ask them.
