I don't know if this helps any, but I'll describe how I'm using spells right now.
The way I think of the spells right now is that I think of them as one of three categories:
A. Spells that need to be aimed and need to be fast. (projectiles in general)
B. Spells that need to be aimed but don't need to be fast. (Platforms and such)
C. Spells that don't need to be aimed. (Rhino, Area of effect around the player, and so on)
"A" tends to go into the left half of my first row which is where my five mouse buttons are as well, and anything that doesn't fit tends to get swapped in and out of that half of the row as necessary.
"B" tends to go into the right half of the first row. I use the number keys with for these spells.
"C" tends to go anywhere else in the inventory. I usually open up the inventory and right click these because they don't need to be aimed anyway. (yes, I open up my inventory quite often
It tend not to rotate the rows to get to spells because I find that, at least for me, it's more prone to mistakes and fumbling.