Originally Posted by
cruxxy
I would disagree. In a perfect setting if you could exactly clip MF at the right time, it may be worthwhile, but in a practical setting, that is impossible. Also, I'm not sure at what gear level you are, but there was some theorycrafting on shadowpriest.com that showed MF clipping to maximize MB was not a dps boost. I think that was at T9 level gear.
Once you reach T9 gear, prioritize your DoTs at all times. If you have a spare GCD, you can use SWD as a filler. Otherwise, just cast a full MF, then MB