Although Fireball is the most mana efficient, once I had built up 5 scorches I went with Fireball>Fire Blast>Scorch>Fireball>Fire Blast>Scorch (basically just to keep dps rolling until Fireblast was ready again). Scorch can be suprisingly powerful with a crit and it's obviously essential to keep the scorch debuff up, you never know when you might get hindered and have it wear off. I speak as a solitary fire mage, I rarely had the pleasure of having someone else to keep scorch up with me so I had to kinda keep my eye on it.
As everyone else has said, dps rotations aren't particularly fun. Of course depending on the mob type, some arcane missiles are often great. If you have PoM a Pyroblast is on the menu every 3 minutes as well, I opened pretty much every fight with it, the 8 or so second DoT just gets nicer as you lay those scorches on and I found taking 6 seconds to cast it during the fight cost me precious pewpewpew time