Holding onto IR for B4B is not worth it. If you just use both on CD, your 3rd B4B and 4th IR line up naturally after 240 seconds. If you hold IR everytime, you only get 3 IR's instead of 4, which - even though all 3 would be buffed by B4B - is a dps loss. 2 buffed + 2 unbuffed IR's > 3 buffed IR's.