I use lots of white sashing in my quilts and have never had a problems. I do try to press away from the white if possible, but often don't. I do take care to trim back threads closely so there are fewer of them to stray under the white fabric. Also, be sure to use white batting, like Warm & White (not Warm & Natural) and this helps to keep your white fabric showing white in the quilt, and helps disguise seam allowances that are pressed toward the white sashing. After it is quilted you really don't notice the seam allowances under the white fabric. A couple of days ago I posted a picture of a baby quilt I did with white sashing and bright fabrics (think the title was something like Baby Girl Quilt for Shower-Very Easy with Charm Pack) that you can look at and see nothing shows through the white fabric.