If I'm using a layer cake I consider it as a 9.5" square and
charm squares 4.5" square and go from there. Easier than
dealing with peak and valleys. I starch all my pre-cuts so
I know they will shrink in width. If a pattern calls for 10"
squares then I buy a fat quarter bundle. For jelly rolls,
I starch and then deal with it as I go depending on the
pattern. But I've never trimmed a jelly roll. I adjust my
stitch width (sometimes twice) or trim later if needed.