Tower has already told you to vacate on to Alpha so there is no confusion on where to vacate.
I would expect you to contact me either just before entering Alpha or on Alpha (doesnt matter as the tower controller left that up to you to decide) but hold short of any other taxiway as your last instruction was only up to Alpha.
There is no point calling ground while still on the runway if you still have a while to go to Alpha, then rather just stay on tower freq until just about to turn in to Alpha.
Its always good practice to remain on TWR freq until clear of the runway but in this case ATC is giving you the option to change freq before else he would have said "Contact Ground on xxx.x runway vacated"
Thats my take.
Last edited by ASD; 14th Dec 2012 at 11:50.