Because you had prefaced your question to Longarmers, I understood what you were asking. One thing of note, is that a lot of times (not always but the overwhelming majority) when you have these variances it is only in the borders which is why so many people responded about attaching borders. If you were to measure the inside dimension of the quilt top before any of the borders start, I bet it would be pretty close to square (that both the sides are within 1/2" of each other and same for the top and bottom. The problem arises when piecers take a long strip of border fabric and "slap and sew" then cut it to be even with the top. The problem is often exacerbated when the quit top center had edges done on the bias, like a quilt set on point where the setting triangles were cut in such a way the bias ends up being the outside edge before the borders are attached. This is why it is so important to measure and cut your borders before attaching them and to pin (or glue or whatever your favorite method is for holding the pieces together as you sew). If this is not done, when the LAer goes to load it on the rack we end up with wavy edges and loads of fullness in areas, that can or can't be worked in with quilting.
If the variance is large like 3" to 4" we usually have to take a pleat (aka tuck) in the border. I always consult with the piecer before doing this. Here is an example. I inserted red arrows pointing to my tucks.
[ATTACH=CONFIG]615855[/ATTACH]
I have also had instances where it was so bad that I had to remove the border, measure, cut it to size and reattach it. I know you aren't the quilt police for these tops, but you could do a great service to the people who are piecing these quilts to show them how to properly attach a border and why. Show them that one side measures one thing, the other side measures another thing then lay the top out flat to show them how this makes for some challenges when quilting on a LA or rack mounted setup. For those who quilt on a table type machine (domestic or extended throat) it does not present as much of a challenge but you will still end up with a quilt that has wavy edges. I have to believe it causes issues if spray basting to get those wavy edges to lie flat.