Gryphon,
In the aircraft I fly (B777), we don't have MDA/DA/DH bugs, they are set electronically, but the function is the same as the manual bugs.
Our SOP is to set them before flight for a possible early return to the departure airport, and then "blank" them until, pre-descent, set them for arrival.
That's the SOP, in an unofficial but useful way, many crews set them to the en-route MEA in case of emertgency descent if MEA is above 10000 feet, e.g. for an MEA of 14500 feet, 145 is set. Useful in the urgency of an emergency descent when finding it from other sources would be a significant distraction.
Other than that they stay blanked out, and memory serves to tell me that they were parked at the 12 o'clock position when en-route on previous aircraft that had these bugs.
Best Regards,
Old Smokey