Floating brake discs

Before last race, I installed new rotors on my floating disc setup. I centered all the bolts and tightened them firmly with a quarter inch wrench, so not a ton of torque on them. After the race, all 4 rotors are slightly loose on the bolts and the discs rattle on the bolts. The nuts do not appear to have loosened. Not having had a car with floating rotors before, I am wondering if this is normal, or should they remain tight all the time?
Thanks for any advice.
Brad