nice guide

You should really spec out of Perseverence and in to Good Hands. The reason is simple, Perserverence is not very good. 4% extra mana regen at 30mp5 is 1.2 mp5. However 10% death timer reduction is 1 sec reduced for every 10sec death timer. In the case of dominion we will say death timer is 20 sec. Thus you get 2 sec extra every death and with lets say 5 deaths in a game of dominion (not out of the question) that is an extra 10sec alive. These seconds are valuable and can mean the difference between a win or loss in a close game. In a normal game that 10% can mean some serious time reduction towards late game.

Also you should spec into Cripple if you use Exhaust with Flash. The crit isn't really that useful.

Great guide! I just got Xerath today and love it.
For me, since I'm not level 30 yet, I have enough to put into offense mastery, but I'm still lacking enough to put into Utility. The cooldown reduction helps soooo much.
If you can keep your mana up with Xerath, you will melt face >:D