"Normally", a mast will be stepped so that the center of effort of the sails is slightly forward of the hulls center of balance.
You can find that center by trial and error by putting the hull in water and pushing it sideways with a finger. There will be a point along the hull where pushing will make it go perfectly sideways, rather than it's twisting.
Putting the CofE slightly forward of this gives the model what is called "weather helm". This means that a gust will actually push the boat into a turn into the wind....which is a safety feature.
If you have differing suits of sails for a boat (for different wind conditions) then the CofE moves and so you need to move the mast too.
Of course, that's the theory from full size boats and there are those on here that will disagree
Cheers,
Robert.