C of g as far aft as practical is the desired loading pattern (for fuel economy), and if 'nothing is said' one would expect a 2/3 - 1/3 baggage distribution to be the 'norm' (if there is one) to achieve a more aft cg. This assumes, of course, correct pax seating in the cabin. In my experience it is unusual for an agent in doubt NOT to ask how to load. How Ryanair cope with their 'free-seating' I do not know, but I guess for them to be less than full is unusual.
Mis-loading is not uncommon. I ?maybe? recall an ?Airbus? of ?Air2000? tipping onto its tail somewhere in Greece a while back. No time to 'search'.
Problems can occur if bags are already loaded (with a rearwards bias) and then a/c is boarded (badly) by agent via the rear doors and rear pax sit down first.