On wet setups, making the gears longer apparently works well. I read this today and it makes sense.
Longer gears = higher top speeds, but slower acceleration & less torque.
In wet conditions, torque = bad.
So, I would hazard a guess that in wet conditions, stagger the gears to minimize torque while still maximizing top speed.
Also, engine braking can effect the choice of gear ratio's that you have. Some corners following a fast section, require that I gear all the way down to 1st, shanghai for example has quite a few. I tend to try and expel extra speed by going all the way to 1st, however if 1st is too low (leftwards) then when I hit 1st, the wheels will not be able to rotate fast enough and, I either blow the gear box or my back end loses traction and I spin out, so I put 1st several clicks to the right to aid engine braking. Conversely, making a gear higher (rightwards) can also decreases it's effectiveness at engine braking. Think about bump starting a car, you push it and put it in 3rd, because this turns the engine over easier than when it's in 1st.
Some gears, not necessarily 1st are sometimes tweaked because it enables a greater advantage to engine braking, rather than provide a smooth acceleration curve.