I am in the process of making a lot of 9 patch blocks and I have been keeping a close eye on my cutting method as the first lot of blocks were not quite right.
Some of the lines on rulers are "thicker" than others and this was throwing out my blocks by just a "smidgen". Once I noticed what was happening the next lot of blocks are making me happy.
Also watch that 1/4" seam - no two machines sew the same 1/4". If you have the ability to adjust the stitch width on your machine do a little experimenting to get it right otherwise draw a line to sew on.
Good luck with it.