'All apllicants i would strongly suggest that you get at least one ATC visit in before your interviews.'
Have to say i agree that this is a good idea.
On the confirmation email for stage 3 it actually says, 'it is not neccessary for you to visit a unit before your interview.' however, one of the first questions in my ATC part of the interview was, 'what preparation have you done for today?' when they learned i hadnt visited a unit i had to explain myself.
Thankfully for me i had a reason why i was unable to visit and it didn't affect the interview but even though it states you dont have to visit a unit i would definately suggest you do so.
I actually think its a slight test to see 'how interested' you are in ATC.