It depends on how much accuracy you need for the parts you’re going to cut. If it’s just decorative stuff, it doesn’t matter. If you make two parts that have to fit together perfectly, you may need to spend some more time on it. As long as you’re happy with the results, any difference in diagonals is fine. Nothing is going to break.
Note I am assuming that frame tubes are nicely parallel though. If it’s hard to move the router to a certain side of the workspace, there’s a chance that the steppers may skip steps there and ruin your work. But if you can move around the router by hand without too much effort everywhere then you’re good.
If you do want to correct the squareness but don’t want to fiddle around with the corners etc, check out this thread. You can make Marlin compensate for non-squareness. (Make sure that the Y axis gantry tube is perpendicular to the frame, and that the X axis gantry tube is the skewed one.)