Since most of us riding fixed or ss bikes are using them in a city, I always recommend (and often swap to) bolt on. There is only 1 reason for this...keeping honest people honest. I hate involuntarily giving away rims and rubber! If you have a bike that travels in/on a vehicle with you or gets parked in a secure spot, then by all means use a QR system. If your just running around the town getting coffee and locking your ride up in random places, then make the thieves work for it and bolt those things on.