When to switch from ground to tower frequencies at controlled airports in the US? 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.



Note: 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.