If you are conducting the approach at pilot's discretion (i.e., no vectors), I think you will be switched to Tower if you are lined up with the runway and are "established" far enough out. If you line up when close to the airport (as sometimes happens with complex approaches) or are too far off the projected glide path, you may not be switched to Tower. If this happens you can tune Tower manually (the frequency was given when you got your approach clearance) and hit hotkey 1. I will sometimes enter the Tower frequency in Com1 standby so I can be ready to tune Tower manually if need be. See also discussion in the following thread:
viewtopic.php?f=8&t=2093&start=10