There is room for pretty much anything you feel comfortable with. That's the beauty of this new mastery system: the only 2 things that matter are that you always have mana to keep casting and that you used all your mana when evocation comes off cd.
Therefore, feel free to use your mana accordingly. If, for example, a dps trinket procs while evocation is finishing, you should burn some of your mana to tag along with that proc. Just remember that, what counts, is that you never run off until your evocation comes off cd, otherwise your damage is going to suffer a lot (much much less if you have Mystic Kilt though).