I'd go along with most of what already has been said -
When there's time for it then the odd - 'Good Morning/Good Afternoon' etc is no problem, when it's busy then you won't get much out of me.
I have said 'G'day' as my 'Goodbye' pretty much all my ATC career and it's never been much of a problem.
What I do have an issue with, is when I either get called 'Ma'am' or I hear a colleague (one in particular was terrible, no names now but you know who you are
) saying 'Good Morning Ma'am' or 'Good Morning Sir' to pilots - as I say I have no problem with pleasantries, but when it goes over the top that's another matter...
G'day!