I would think that for pvp microing blood tap when you need a rune or will cap on charges would be best. That way, you can have a rune always ready for a silence, dark transformation/pillar of frost, or sudden soul reaper if your target gets focused.
But if you were going to macro it, I think thats the correct way.
The only thing I macro BT to is Necrotic Strike. But why would you macro it to DC in the first place? DC costs no runes...
probably so he automatically activates it when he has enough stacks right after a DC.
I agree with Vereesà though. Blood Tap should be used when you need it, not when it's ready to use. it's like you don't activate AMS or AMZ on CD, you use them when there's an opportunity to get benefit from it, and you should use it smartly. no use using it to convert an unholy rune, and if it converts a rune that will activate as a death rune, you've also not used it optimally.
Why? Because it's just as efficient as runic corruption but gives a death rune, which is nice in PvP for necrotic strike. Macroed BT is better than RC for most applications. It's obviously worse than non-macroed BT, if you're willing and capable of micromanaging your stacks.