Isengard is my main list, though I dabble with Misty Mountains (sans Blackshields!) and recently Angmar (well Buhrdur's warband allowing lots of Cave Trolls, that's fun)
Generally yes, when in doubt take Saruman, He is a one man Artillery unit. This makes him a prime target for enemy duellers, but a small amount of care can keep him safe mostly. I agree with the spell order too.. Though with a proviso: Exsicate is better against lighter armoured large formations, while Bolt of Fire is better against higher defence smaller formations. i.e. Bolt of Fire barely tickles a 6co formation of goblins, as they are so cheap they soak up the damage, but exsicate will be devastating.
Companies of 4 seem to work best for Isengard for flexibility and manouvrability, and packing still a mighty punch and won't be wiped out quickly. I do though like to bury Saruman in a 5 or 6co formation of Shield uruks, as it gives him a good 'platform' from which to sling spells, that won't be easily cut from under him by enemy fire.
Phalanxes... urg. These are good to 'fill out' an isengard list, but in my opinion arent worth their points, as someone mentioned above, they do look cool, but all they do is make enemy cavalry bypass them and hit other formations. Crossbows are faaaaar more effective, i take as many as I physically can
Command ---- captains.. mmm can be useful. In general the WotR rule of thumb is 25 points = 1 Might point for heroes or captains. So captains provide 2 might points, a slight increase to atatcks and courage, and added speed as you can At The Double! If you already have Thryden, Saruman and possibly Lurtz, you might not need too many captains.
Banners though are far less clear cut. weigh up... 35 points for one more company of Uruks... or a flag on a stick that allows rerolls on charges and panic tests? Well I like to take one in my list as it looks cool to see the White Hand flying, and i place it in the formation most likely to need it's bonuses.
Drummers - 15 points for +1 speed... well if you have those usual pesky few points left over when trying to make 1000 or 1500 points and not enough for an extra Co of Uruks.. then by all means take a drummer. Again place him in a unit that needs it, which isnt likely to be a formation of Crossbows...
Shaman - Sigh... More Ruin spells are cool. but at 100 points for mastery 1 and 1 might point when you could ally in Druzhag or Kardush for the same money... or a Nazgul at 125 points... Lets just say unless an Isengard player rigidly takes no allies, and even then due to the high cost you will rarely see a shaman on the battlefield in WotR.