Fabric often distorts in the finishing processes. Those tiny holes along the selvedge edge are made when it is stretched on the 'tenter hooks' of the finishing tables (this is where the phrase 'Being on tenter hooks' comes from). If the fabric is skewed when doing this (the cheaper the weave the more likely this is to happen - and lots of quilting fabrics are not great quality) it will be set off-grain.
You can do a lot in the pressing to set it straighter, so dampen it again and press and pull back to where it should be. I do however find it always has a tendency to creep back to how it was set.
So how to work with this? Well you can cut everything out singly and pretty well on grain (even though the warp and weft threads may not be set perfectly at right angles to one another), so no double or four ply cuts. Alternatively you can put the fabric selvedge to selvedge and cut off the uneven bit and work with the units slightly off grain. How much this matters depends on lots of factors - you and how much you mind this, how much the fabric frays (there is an argument that a slightly off grain fabric will fray less thank one on the straight grain), your skill level, how easily your machine allows you to feed work in without distorting and shifting the plies, whether this is visually detrimental to the look of a print, how complex your patchwork design is to sew and more.
Remember the patchwork top is well secured by the layering up and stitching to the backing fabric and wadding so this should be taken into account too. Make a sample and see how you get on and then you will know how to proceed.