Mig? Arc? Tig? What sort of welding?

Yes, always tack up first, when choosing where to weld, always think of contraction - the two sides of whatever you weld WILL be drawn together as the weld cools, no two ways about it, it will happen.

On a box frame I would tack up and check all square, then weld the inside 90 deg angle on opposing corners first. Next I would do the outside edges followed by the two faces. Welding the inner/outer sides first should help resist the tendency of the frame to buckle if the faces are welded first.

Test your settings well on scraps first.