As the miniature is not an order it actually can be placed where the 'owner' wants to place it.
Orders have stipulations as to where the order shall be placed.
Crown Prince Mary also chose to wear her miniature as Princess Marie did - at the wedding of the Swedish Crown Princess!
I think the reason why HM Queen Margrethe always wars her at the same spot - is that HM wears the Grand Commander Cross and that has to be placed where it is placed - and it would look a bit strange to put her miniature on the others side then - and where should she place a brooch then?