the total positive reputation available is limited, while the negative is not.
So the usual (i. e. the one that I used in the one of my two finished games where I was able to pick up Light/Dark) process would be to first be the good guy/gal and just amass the positive reputation from all the quests, then get the master light promotion, then commit some atrocities (sell human bones, wreck the Mire of the Damned with some Armageddons (I love that feeling!), use the Baa head in the superior Baa temple, etc. etc.), get your master dark promotion, and then pretty much ignore the reputation until the end of the game.