In fields badly infected with sclerotinia, swath when the healthiest plants are ready. These plants will provide most of the yield. Swathing early will not preserve seeds in badly infected plants — those seeds are unlikely to mature no matter when you swath.
Swathing healthy plants before 30% seed colour change, for example, may not allow those healthy seeds to mature and fill fully. Keep in mind that when swathing fields with sclerotinia infection, swathing in a little rain or in the evening dew will limit shattering of those mildly infected plants that may have some yield but have matured prematurely.