Originally Posted by
sykerocker
On the rear, you put it on the left to have room to turn the lever. Putting it on the right would be a nice way to pinch your knuckles on the dérailleur.
On the front, you put it on the left because it looks nicer, matching the rear.
Well, that's the best reasons I've ever found.
There's more to it than that, as you can always adjust the tension by turning the nut, rather than the lever. The practice stems out of race situtations, where having a flat and how quick you can change the rear wheel is crucial. Placing the lever on the drive side would cause all sorts of potential problems. While inserting the wheel, the lever itself is likely to get caught on the derailleur or cable housing, causing delays in getting the wheel in place. The lever also has to be carefully orieneted once in place to avoid fouling the derailleur or cable. In a worst case sceneario, it may be inadvertently closed onto the rear derailleur andd thus may not be fully closed. In a race situtation you do not want to worried about thinking about such things, as they rob valuable time.
Another race driven practive relating to QR skewers is the convention of having the levers pointing into the middle of the frame. If a rear lever faces backwards, or a front lever points forwards, there is the possibility that another rider's wheel could become placed between your wheel and lever. If that rider tries to steer away from you, he could unlock the lever and/or crash in the process.
The levers particularly the rear, is also often pointed upwards, as a rear lever that is horizontal or pointing downwards would be below the chainstays and could catch and come unlock when unloading bicycles from car racks. This could be disastrous if it occured during a bicycle change in a race.
The practice of having the front lever on the left is, as far as I'm aware, simply to match the match the rear. However, there is practical standpoint in the modern era. Most computer manufacturers instructions state to mount the magnet and receiver on the right fork leg (probably because of the convention of mounting the QR lever on the left). Having the lever and receiver on the same side would increase the possibility of knocking the receiver out of position. If nothing else, keeping the lever consistently on one side ensures you replace the wheel so that the magnet is on it's correct side too.