You don't want to bind Scorch to any button that you have to press with your index, middle, or ring finger because it can limit your mobility. For example, in case you use WASD and you bind it to F you can't go right and cast Scorch at the same time. Never bind anything that you might need to cast while moving to a finger that might be busy.
I have given up WASD a long time ago because of the limitations it comes with. I'm playing UHJK.
It gives you many more "pinky buttons" for smoother gameplay on the move and more buttons to bind overall. It also allows for convenient double-binding of all keys via the right Alt-Key that you can then press with your left thumb, giving that finger something more to do than just jumping.
My system is like this (German keyboard layout, MB4 reassigned to Shift):
Class 1 Keys (1 press, movement independent, reach 1): G, B, MWup, MWdown, MWpress, MB1, MB2, MB3,
Class 2 Keys: (2 press, movement independent, reach 1): Alt+G, Alt+B, Alt+MWup, Alt+MWdown, Alt+MWpress, Alt+MB1, Alt+MB2, Alt+MB3, MB4+G, MB4+B
Class 3 Keys: (1 press, movement independent, reach 2): F, V
Class 4 Keys: (2 press, movement independent, reach 2): Alt+F, Alt+V, MB4+F, MB4+V
Class 5 Keys: (1 press, movement restricting, reach 1): Z, I, L, 7, M, ,
Class 6 Keys: (1 press, movement restricting, reach 2): T, O, ., N, 6, 8
Class 7 Keys: (2 press, movement restricting, reach 1/2): Class 5 and 6 + Alt and class 5 and 6 + MB4
I also use a foot pedal because it's hilarious to have TW bound to the gas pedal, Ice Block to the break and switching gear sets to the clutch.
Admittedly, the only mage spec that I needed class 7 keybinds for is a currently unviable Pet Ray Frost build with Comet Storm, Frost Bomb and Ice Nova and yes, I really do hate clicking stuff.
It still helps to think about these limitations when assigning keys to spells because it will make things a lot smoother when it really counts.