i am installing milgard classic vinyl windows (nail fins).
to make sure that the window is level, i might have to add
shims before nailing the bottom flange.

can i place space shims between rough sill and the bottom of the
window? will using shims here cause the window to sag? should the
shims be placed only at
the corners?

once the window is square and nailed in place, can i remove all the
shims? or should i just leave them alone or do i need to nail the
shims as well?

thank you in advance.