Can't speak for the other operators or fleets, but on B738's for Scareways, when Ground says "Contact tower when READY", we try and do exactly that, or otherwise contact tower and tell them how long before being ready.
It's a bit unprofessional from the pilots point of view to make ATC wait. When contacting tower, after being told to "WHEN READY", ATC shouldn't have to ask "are you ready for an immediate?". By contacting tower, the a/c should be ready, or otherwise say if they are not surely...