Is it best to have the front wheel release lever in front of the fork, tailing the fork, or aligned with the fork? How about the rear; ahead, behind, or alingned with the chainstays, seatstays, or between the two?
Quick release angle on the front skewer shall be an upward angle which tightens just aft of the fork and the rear quick release shall tighten at an angle that bisects angle between the seat and chain stays. It is acceptable, however, to have the rear quick release tighten upward, just aft of the seat stay, when the construction of the frame or its dropouts will not allow the preferred positioning. For Time Trial bikes only, quick releases may be in the horizontal position facing towards the rear of the bike. This is for maximum aero effect.
For disc brake bikes specifically, you want to have them positioned in the way that makes them easiest to open without touching the discs. For the front this is easily achieved by putting the lever on the right (in whatever orientation you fancy), but you can also have it slightly in front of the fork on the rotor side (behind also works if it's not interfering w/ the caliper). In the rear, I find that just slightly below the chainstay works best for me, making it easy to brace against the chainstay for both opening and closing.
Mechanically it makes no difference, since the quick-release work any way you position the lever.
The only reason to adjust the levers position is for the ease of use. The best way is to position the front lever slightly behind the fork (not aligned along the fork, otherwise you cant press the lever down completely and its hard to open because you cant get between lever and fork with your fingers).
Same for the rear: between the two makes the lever accessible most easy. You can press with the thumb on the chainstay/seatstay and open the lever easy.
Personally, I think it depends a little on the design of the quick release lever. I put my front one pointing upwards aligned with the fork. However, some quick release designs will actually contact the fork before they are fully closed. In this case I would place it slightly behind the fork. For the back quick release, I position it between the seat and chain stays. Again if you have something in the way, you'll have to adjust parallel with the seatstay or even behind it a bit if something is in the way.
I've seen some people position them both facing backwards. The idea is that they are less likely to catch on something and open as you are riding. However, I think this makes them more likely to be opened by some other factor when not riding because there is nothing guarding them. This would be fine if you checked them before each ride, as we probably should, but most people probably aren't consistently checking their quick releases every time they ride. If they are in a position that they are least likely to be opened accidentally in most situations, then that is probably the safest.