I cut my lower X-brace legs off and throw them away....
Some great posts above with David's and Ken's being excellent in my view.
I don't tuck either and also being a repair guy my life is easier when they are not tucked but not by much. I'm also after a freer top and not only don't tuck but I also thin the edges as Ken mentioned.
Now here is some more food for thought. If I know where the instrument will end up, who is the client, I might consider tucking if it looks like the thing will be subject to abuse. A hard knock of the lower bout can free a brace unexpectedly but when tucked this is theoretically perhaps a bit less likely but still happens.
OTOH if I know that the instrument will be played by the sort who is less likely to leave it on the sofa in a crowded dorm room during a beer drinking contest... I won't tuck.
In my experience I have seen both come loose but not without being well provoked. I also agree that what is perhaps more important is that the brace is well clamped and glued, glue protocols are observed for the glue of choice, etc.
I do tuck the uppers and of course the UTB too.