I don't know how much you will get out of a general discussion about magic, there are so many variables each army has to consider for it to be of much use and not just general nonsense.
That said, I would say that unless the army (Dwarfs basically) or theme (Khorne basically) demands it the best thing to go for is a lvl 4 and then if you feel you want a higher presence you add in a lvl 2 or two.
For my Beastmen I usually go with one lvl 4 and, depending on what else I want to field, one or two lvl 2. Bray Shaman are quite tough so they can withstand stuff normal wizards generally wouldn't like and Beastmen demand a certain level of magic to be involved given their lack of both shooting and protection and, fluffwise, their nature. I usually go for Lore of Shadows but on occasion I like to field a lvl 2 in a chariot with the Hunting Spear and then I just give the other lvl 2 Shadows while the charioteer and lvl 4 goes for Beasts since it means I can often get double bolt-thrower action from my chariot and it does after all carry a bonus when used on all Beastmen units. What I want from my magic phase is buffs to my own units so that they can remain on the table and strike hard when they do strike, some hexes are nice but generally I prefer to increase the survivability of my own units since they are quite fragile but gain a lot from most blessings.