The canopy should ALWAYS be released before releasing belts. This is common in the aerobatic world, and ensures that the pilot can physically release the canopy before bailing out. Aircraft loading may put the pilot in a state where they are unable to release the canopy if not strapped in (I.e. negative Gs or high side load).
It is good practice to have this routine down cold…every time one exits the airplane with a parachute, running through the bailout process (canopy, straps, bail, look, reach, pull with both hands) can be very beneficial to solidify good habits.