During my RT training, the RT instructor - active ATCO, advised to make the initial distress/urgency call on the active frequency, as someone was more likely to hear it, especially here in the UK.
He also pointed out, that D&D do actively encourage practice pan calls on 121.5 so they can check that the fixing kit works and use it for training. There was also an excellent article in Pilot magazine last year about this very subject.