Crime happens when you have over 1200 unused S/D in a fief
True as a guideline, not true in all cases. The mechanic works so that it is 1000+prosperity = crime threshold. So, if you have a fief with 500 prosperity, you need not fear crime until over 1500 s&d builds up. That said, as a traveler/trader, you can't see the prosperity of a fief (directly on the map anyways, you can see it if you check the fief list) so if you're lazy just don't go entering any fiefs with over 1200 s&d. Unless of course you're right next to it, ready to enter, and it already has 1200+ s&d yet no crime has built up yet. Then you can stay their safely until the next hour rolls around, in which case you may be at risk (unless you of course used up the s&d during your visit).