I have heard that you can switch to the tower frequency once you are at the runway hold line and are ready for takeoff without getting permission from ground control to switch frequencies. Is this always true or does the procedure vary for different airports?
While I acknowledge the answer by GdD to not change frequencies until instructed, that is at odds with my experience.
When I call Ground, then request and receive taxi-instructions, I stay with Ground to the runway hold short line. I've never had Ground give me an explicit instruction to switch to Tower.
When I start my run-up checks, I switch to Tower (unprompted) to start hearing what is happening in the airspace. When my run-up is complete, I call Tower with a "ready for takeoff".
I've never had Ground, Tower, or any Instructor tell me differently. I've never observed another pilot wait for an explicit frequency change instruction from Ground, and I believe that is the way I was taught.
this only applies when you've taxied to the hold-short with Ground. If Ground tells you to switch earlier, do it. If you want to switch earlier, request it.
I have found this AIM 4-3-14a which I believe answers my question:
AIM-4-3-14a: Pilots of departing aircraft should communicate with the control tower on the appropriate ground control/clearance delivery frequency prior to starting engines to receive engine start time, taxi and/or clearance information. Unless otherwise advised by the tower, remain on that frequency during taxiing and runup, then change to local control frequency when ready to request takeoff clearance
This seems to suggest that it is OK to switch to tower once you have completed the runup and are ready to takeoff at least in the US. Also interesting here is that it says that you should contact clearance/ground prior to starting your engine. I have never heard of that before.
If you are under control of ground you only switch to another frequency when directed, or are granted permission. They'll tell you "Contact tower on xxx.yy", for example. You might ask to change, say you only have one radio and you want the latest ATIS, in which case you request to change, and wait until they give you positive indication that is okay. You may need to shut down your avionics to start up your engine, especially common in light singles, in which case you ask permission for that as well.
If ground goes off of the air or you have trouble contacting them you can change to a different frequency for a radio check, or if there's some sort of emergency requiring changing frequencies to resolve then do so, it's hard to think of a realistic case for that; if you have an emergency on the ground then ground is best placed to help you.