I like to research Evocation first (well, depending on my mage paths; normally to level 1 or 2) just in hopes of getting heroic mages. I love them! Quickness, precision, research, stamina - they can be a huge boost, and are fun too.
I send out my first mages ASAP casting only evocations on that hope. Fire and Earth are the best, and often become heroic... of course I usually get heroic strength, but often it works
Taxes: In my experience, 10% equals 2-3 points per turn, in low or neutral dominion for a province with low unrest. Thus, I set a new province with 13 unrest at 30% taxes.
However, that may be conservative (or based on an older version), as in my most recent game, provinces with very high unrest were cleared of roughly 33 unrest per turn with 0% taxes... and this was fairly consistent.