Mind Blast is something that should be used frequently, usually used with at least 1 shadow orb to refresh empowered shadows. Whenever you refresh empowered shadows, you can afford to cast the next mind blast that's coming off CD without any orbs, since by that time there is still enough time to wait until the mind blast after that for refreshing purposes without jeopardizing the empowered shadow's uptime. However by doing so, you are slightly gimping your chances to accumulate any shadow orbs from mind flay, since the time spent casting the 0 orb mind blast could have been 2 ticks of mind flay, hence 2 chances to generate an orb. However in the long run these 2 extra chances are relatively quite small, and usually taking that risk (as opposed to playing safe) to cast the 0 orb mind is more worth it in terms of overall dps. However again there is an exception to this risk in the circumstance where dots are close to falling off soon. In that case I'd rather play it safe and NOT cast that 0 orb MB because I don't want to risk refreshing those dots in case I lose my empowered shadows at the same time.