If you use a base square (or any shape really) to sew them to it will not matter if the grain is straight or not. Many old time quilters made tops this way to use up all of the scrap fabric. Just make sure the base fabric shapes are on the straight of grain.
The stores sometimes wrap the fabric wrong onto the cardboard and it is off grain. In this case sometimes a good tug corner to corner will bring it back into line. Sometimes you will need steam to accomplish a straight of grain set. In that case, pin it to your ironing board straight and lightly steam. Don't scrub on it, press from the back to help keep the homespun look. Allow it to cool in place to set the fabric on grain.
Last edited by RedGarnet222; 02-20-2018 at 04:12 PM.