Also to consider ... I have found that it really doesn't make a difference unless you want to keep to the exact measurements - ie an 80 X 80 quilt WILL indeed be 80 X 80. If the end result doesn't have to be exact, then the most important thing is that ALL seams have the SAME seam allowance. This can take some adjusting from piece to piece if your using different types of fabric - or even different manufacturers. A Moda cotton will have a different end result than a Batik. Heck, even some Moda's are different than each other.
Also, you can probably still use your 1/4 foot guide rail - just move the needle position over to the left a smidge to accomplish the "final" 1/4" seam allowance after ironing.