If you only have a lot of excess fabric in one direction, I would cut it down. Applique work, especially circles, typically draw up the same amount of fabric from the width & the length. If you cut some off in advance, you can get a larger single cut that you could use in the future. Also, I personally find it easier to keep track of the layout if I don't have to try to picture it the same length, but much narrower.
I sell baby quilts &, in general, for whole cloth applique, I typically add 5-6" to the length and width of background fabric to account for needle turn work, hand embroidery & dense machine quilting. With whole cloth backgrounds, I buy a 60" cut of fabric (1 2/3 yd), which gives me a finished size of 36x54" (42" = WOF). At that size, I do not cut my background fabric until at least after I've finished the applique. At that point, I either physically square up my quilt with a rotary cutter so the sides are even & proportional (usually that's 39x57") or, more commonly, I measure from the center out (18" right & left of center; 27" above & below center) and mark the lines so I have a straight edge to work off of when quilting. Once I finish quilting, I cut to the desired finished size.