If you're looking for that info on racing or acro quadcopters, a good starting point would be to look at what types of tilted camera mounts people are using. Just as Neil said, it can vary by a LOT depending on the type of flying. The tilt determines how fast you're flying in that direction, so it's anything between anything over zero, and can surpass 90 for a bit if you're losing altitude. The thrust to weight ratio will determine how close you can get to 90 degrees before losing altitude. 22.5 and 30-degree camera mounts are common for acro/racing, so that should be somewhere around average angle during that type of flying.
Yah ultimately kind of up to your flying style, totally flat is easiest for take off and landing since can see the ground but when tilted forward flying then you're mostly looking at the ground instead of what's in front of you. Long story short the faster forward you want to be flying the more you'll want the camera tilted up to compensate if you feel you are always flying too fast just to see the ground then reduce the camera uptilt, just something you can toy around with to get different look or flight style too.