Once the fabric is pieced into the quilt and quilted there is little stress on it to pull wonky again.
You can pull the panel close to square and even it it is still off a little, cut on the square you want to cut, not the fabric grain. Starch helps a lot to prevent fraying on wonky fabric.
When working with wonky panels, the tugging in opposite bias corners, back and forth a little at a time with firm but not aggressive pressure and stab pin it to an ironing surface, dampen with starch and press should hold the fabric in place as a preparation to cut it square. You have to make the fabric move to your will. It will go where you want it, if it is not overly wonky.
Anxious to see your project!
peace