Nice bag!
If it's a printed plaid that is off-grain - after washing it to see how the fabric wants to go - I feel that one has two choices:
1) Cut with the print and the fabric is off grain
2) Cut with the grain and the print will be off
I've learned to be very aware when buying printed stripes and plaids. They are not possible to straighten.
Woven plaids and stripes can be worked into alignment.