It's okay to cast mind flay but only if void bolt not will be available during the gcd it takes to cast mind flay. Get into the habit of using VB>MB>VB>mindflay. If you try to sneak a bit of mindflay between your mind blast and 2nd void bolt (VB>MB>MF>VB) you'll actually lose dps and time spent in surrender. Better to just not cast during the half second it takes for the next void bolt to come up.