I'm well out of ops now but back in my day it was no big deal. If I was initiating the call to an aircraft on stand, it was probably to ask/tell them about something that might be advantageous and if they don't answer, they miss out. If it was bad news, there'll be plenty of time to tell them later. But I also look at it from the pilots' perspective - there can be a lot going on as the flight preps for departure and I quite understand that turning the radio down if you're not expecting a call might seem like a good idea. Then again, how do pilots/drivers feel when they call ATC and don't get a reply immediately - there were plenty of times that I prioritised something else over answering an RTF call straight away? But I'm with terrain safe on the taxying thing, once an aircraft is 'active', I would expect a listening watch from an aircraft at all times.