I think it would be cool to have this etched on my own swords, perhaps in Latin. 
I was thinking the same thing, but I never learned what sheath was when I took latin in school, so I turned to a translator. Uh... you may want to rethink it, or we may want to come up with an alternate word for sheath....
"Traho non sin causa; me vagina ignobil" (this is my best shot at it, using "cause" instead of "reason" and looking up what "sheath" is)
"Eripe me non sine causa; me vagina ignobilis" (this is what google translate gave - which is odd because i thought eripe is "save")
You could use "return" instead of "sheath", and "Draw me not without reason; sheath me not without honor" would be similar to "Eripe me non sine causa; me redire ignobilis".
It's a very cool phrase. I vaguely recall something similar in an asian martial arts movie where something was said along the lines of "Don't draw your sword in anger, but when drawn, don't sheath it until your enemies have fallen" - I know I've butchered the exact wording, but that was the gist of it, and it was an older movie.
EDIT: Doh! Ninja'd by Sir William. I also looked up scabbard, and it gives the same word. I don't recall ever seeing "mihi" in latin at school.
