You said nothing about knees. You said slung low. I play fairly slung low and I have no problem keeping my technique up. Even if you sling it low, you can hold the neck up high and do 1 finger per fret on most of the neck.
I do not want to go into an endless argument, but down below a certain point, you are compromising your technique by slinging your bass low.
What I mean is that if you have your bass at wais level or so, you can't play crazy ultra fast licks all over a the neck as effortlessly without having to have weird hand placment.
However, in real life, nobody needs to have all that reach all the time, and most of the basslines are played in the lower portion of the neck anyway.
Again, i'm not saying you can't have a decent technique while slinging low, it's just not optimal, especially when practicing and for a new player.
Looking cool as nothing to do with where you strap your bass, You just have to make it work for you, for your look, there are plenty of famous bass player in rock bands that strapped their basses high, and still look very good.
Look at rage against the machine, look at the pictures posted earlier, protest the hero and the band foals, they all have a look that work with having an instrument strapped high.
In the end it's about what your band is looking to acheive as a global look onstage, you all have to fit together. That doesn't mean you have to start practicing with your bass strapped in such a way that you are incapable of playing some more adanced stuff.