MB 17: To sleep in them - And also to give them to a non-Jewish launderer. However, some are stringent in this matter, and say that one should do as mentioned above.
21.4: One should be careful when he wears a talis (18) not to let its tzitis drag [on the ground].
MB 18: Not to let drag - Some say that the reason is abasement of the mitzvah, and in addition the tzitzis are liable to be made invalid [by snapping off, etc.], and so he should lift them up and stick them in his belt.