I think the cashing out threshold is an individual decision. If you started from scratch by winning freerolls why not build it as high as you want before cashing? You didn't invest anything but time so if you lose it all no big deal but if you hit that one big payday in a tournament with it you can cash out some serious coin then. A real rags to riches story come true!

And with the hole cards I usually get in money MTT's, it would definitely be a "rags" to riches story.