In years of riding French bikes, I have never had a fixed cup come loose. I install them with the Var 30 tool, which allows you to torque them down real good-like. I'm aware it can happpen, that seems to help stop the problem before it develops. I have never used loctitie on a fixed cup.

Interestingly, I've repaired several fixed gears in recent years that had the English thread fixed cup come loose. Same basic problem, I think: the back-pedaling and resistance stops work the fixed cup loose on English-thread bikes. I've tightened them up as well with the Var tools, no further problems reported. The most recent was brought to me by a co-worker at the art center I teach at, a local shop had quoted her 75$ on the job (I did it for free).