I am fairly new at quilting to and I always thought that it was a proportion thing, you know dividing into thirds etc but I soon realized that there are no rules or formulas that are considered standard, it is all down to personal interpretation, some large blocks have skinny sashing some smaller blocks have wider sashing, as has been already said, auditioning is the best way, you never know something unexpected could jump out and say, 'pick me'.
On my current quilt for my DH he said he wanted yellow sashing, I thought yuck, I auditioned it among others and guess what, it looks great, so what do I know? Lol
Gal