No, you should not. Mind Flay is regarded as a filler spell, which means it is only used when nothing else more important can be cast. You typically want to start thinking about re-casting Mind Flay about halfway through the channel to make sure that if you get a Dark Thoughts proc you have enough of a cast left to fit in that global cooldown. Mind Flay has a base channel time of 4.5 seconds, which is reduced by Haste. The global cooldown is imposed on most spells in the game and it prevents you from casting multiple spells at the same time; the base global cooldown lasts 1.5 seconds, which is reduced by Haste. As you may be able to tell, the duration of Mind Flay is triple that of the global cooldown, and so as a rule of thumb you can say that Mind Flay has a duration equal to 3 global cooldowns, or 3 globals as they are popularly referred to. They both scale with Haste in the same way as well, so no matter your Haste, Mind Flay will always last exactly 3 global cooldowns worth of time. The above is relevant because Mind Flay deals its damage over 6 ticks, over the course of its duration. Since we have established that Mind Flay does its damage over 3 globals, we can state that Mind Flay does 2 ticks of damage per global. This means that there is no loss to cancelling Mind Flay early, as long as you do it after every global and not during a global.