cheetah..the reason for asking 'say again freq' may be that you confirmed it,noted down,changed it and got no reply ...because you've received a wrong freq or you missunderstood it and the atc failed to correct your readback....so please forget the egg-on-face argument.
The only reason to write down the freq is that it can happen to change by itself or by mistake.I've seen once the freq changing by itself,due to some quartz
problem (i've been told)..so you better have it written somewhere.
But even so,you have 121.5...no shame in asking there for some help.
QSM also...
Brgds.
Alex