during combustion window: it depends on how much haste your have/reduced caste times, if you have heroism/4p set bonus active during combustion and you get a bracer proc off you should definitely cast it, otherwise ignore it until you have finished your combustion window opener and then look to hard cast it.
outside of the combustion window you want to use them as often as possible when the situation allows it, there's a few times on progress fights where i have had to let a proc die due to dealing with certain mechanics so if the situation allows it, get it off then go back to your normal rotation.
as a general note also, make or find a weak aura to track your pyretic incantation stacks and wait until they are high, 3-5 is ideal then cast pyroblast so you get the most damage out of each cast but this is pure min maxing and isn't always viable due to fight mechanics etc.