In my preferences I have checked:
Enable auto mark QSO QSL_S fields as: SB
Mark only the first QSO, first band QSO, first mode QSO
Now when I enter a new QSO with a station I never worked before, I check its qrz.com profile if it is ok to send QSL via bureau, and if not, I clear QSL_S to empty. When I update other fields like reports or time, QSL_S gets reset to SB again (not always but pretty often).
It would be great if the automarking would be used only for the default value of QSL_S and then the field would stay at the manually set value.