Dispel is good when you have your troops buffed, I max it maybe not ASAP but quite fast. I don't like returning to castle, and I use sacrifice a lot because of leveling and leadership bonuses
I don't have trolls, instead I suck from Rune Mages (buffed to max with Mind Runes).
Yes, everyone has a strategy (or couple of strategies) which suits him/her best. I noticed that of three spells I use most only Phoenix is Order Magic. Oil Mist and Flame Arrow are not, just like Trap, Fear or Stone Skin. But Phoenix should be maxed, and these other spells work almost as well at level one, IMO.