Depending on your mastery, you should use glyph of frostfire bolt. Increases frostfire bolt's damge by 15%(now same dmg as fireball) and puts a dot on the target that stacks up to 3 times. Now your main nuke will be frostfire bolt, not fireball. Im at 13% mastery and i do much more damage spamming frostfire bolt as my nuke then spamming fireball as my nuke(even with 5%crit glyph)