Have just been studying the UK MATS Pt 1 which states;
"To indicate an emergency condition pilots are encouraged to select code 7700 as soon as is practicable after declaring an emergency situation........"
However I still stand by my previous - don't be afraid to whack it on first
Would partly agree with Spanks, If you are carrying dangerous cargo we need to know. But I, for one, won't bother asking if you are or even for your SOB. There must be much more important things for us all to be doing!