It always seemed to me that when the pasuk says "and you shall see it and you will remember all the commandments of Hashem, and perform them...", that one should wear their tzitzis un-tucked, so that one will see them throughout the day and remember his obligations to Hashem.
In Yeshiva, I was told by other bochurim that this is not the case, and that the pasuk is only teaching us that tzitzis is worn during the day (i.e. when you can see them), and not the night. I was even told that one
should tuck them, that when the pasuk says "and
you shall see it", this means
you and not
them, i.e. the non-Jews, and therefore one should tuck them in when going out on the street. It seems also that historically, when Jews were being persecuted, then it made sense to tuck them in as a life-saving protection, and that this then became the custom.
However, I have come across the following Midrash to support the un-tuck viewpoint:
The Midrash Tanchumah, Parashas Shelach, Sif 15 says:
"And it shall be for to you for tzitzis..." - That it it should be visible. ... "And you will see it and you will remember.." - seeing leads to mindfulness, and mindfulness leads to performance, as it is stated, "In order that you remember and perform all My commandments." Why? "For it is not an empty thing for you."
It would seem from here that to properly use tzitzis the way G-d intended them to be used, is to wear them out so that they are visible.
The argument could be made that when the pasuk says "see it", this does not refer to all the strings, but only to the techelis string(
it, and not
them). Therefore since most people today do not wear techalis, perhaps there is no need to un-tuck.
However, I still think that if a Jew lives in an area where there is not active persecution, and is not roaming around in dark alleys, that he should wear the tzitzis un-tucked. I don't know if this is right or not, but it seems to me to make sense.