After some testing it seems that the next rules apply to Disco-dps
Smitex5 (to get evangilism maxed)
then priority: Penance>Holy Fire>Smite
But, if you happen to get your Holy Fire of CD, with only one GCD left for next penance, it seems to better smite,penance, holy fire then.
Glyph of Penance
Glyph of Smite
Glyph of Accuracy
Problem is, that I tested this with a lvl75 on a lvl70 dummy. And Penance and Holy Fire have a 18% miss chance normally....
So, maybe Smite-spam is the way to go.... mmmm?
Why am I even thinking about this?