Check for flat faces?
 

How do I make 4 edged polys flat again if I've warped them through vertex manipulation?

I hope that makes sense because I don't know how else to put it.

My model is going to be folded into a cardboard model so I want all the faces to be flat, so that the cardboard won't have to bend or twist.

It's easy if you'll have them exactly vertical or horizontal, cause then you can go into an orthographic view, select the four, and holding x, move snap them into a line (I think you may have to deselect maintain offset). I guess you could snape them into a line and then rotate the group of four if they're supposed to be at an angle. Hope that works for you.

The problem is when they're not flat and snapping doesn't work. Is there a way to change the snap plane to something besides global?

Also, rotation doesn't work because I can't figure out how to move the rotational axis to be in line with the plane.... like you can with the move tool (hold down w, then hold left click for the menu)

if the face you want flat is on an angle to the world then what you could possibly do is use the scale tool in one axis, the verts will align to a flat plane and then use the rotate to rotate it back to the orientation you want

Hmm, that technique has promise.

How do I move the scale widget (by pressing insert) to be somewhere besides a vertex? If I have a 4 sided polygon I'd like to have it right in the center so that the scale doesn't move the vertices all around