For cropland, allowing for the margins is probably the right approach if they’re big margins, or if they contain vastly different things from what’s in the field (like trees or scrub). I’m used to mapping mostly sheep pasture, where there are no margins!

If the fields are used in rotation there’s no great answer. You could either invent some tags to describe the rotation, or tag the fields as they’re seen in aerial imagery. That will at least give a snapshot of the rotation at one point in time, which is likely representative of the bigger picture.

Cheers :)