Enough that there's a rule about it.
And I've issued a DQ for loading before "make ready" once. Loaded table start. Competitor approached the line, unholstered and began inserting a magazine before make ready. Earned them a "Stop, Unload and Show Clear" and no more shooting that day.
There's also lots of it happening at the trunks of cars where competitors step out with concealed carry guns and take them off their belt, etc., to put them in the car (or the reverse at the end of a match). That's a loaded gun, too, and falls under match rules.