The square knot is a binding knot. That is what it was intended for. In that use it is good. It is NOT however, intended to bend (join) two ropes together. It is absolutely terrible for that purpose, whether kept under tension or not. The fact that you've suggessfully used it for climbing is nothing more than luck. Each time where you lucked out before you could have just as easily been a Darwin Award winner instead. I am not saying this to sound belligerant, but the square knot has probably killed or injured many people by being improperly used as a bend than any other knot. But don't trust me - go read any reputable knotting book and find out that every knotting expert says the same thing about the square knot. It is neither a strong nor a secure knot. Its good enough if you're just tying a bundle of newspapers together. But don't use it for anything more critical than that.