When do I drop Grit and Drk Controller questions.

[Aiio]

Personal experience only... (AS4 cleared 3.0, AS7 cleared 3.2 in the first month of release then took a break)
Dropping grit is used generally in one of two situations:
1. OT
No reason to reduce your DPS if you aren't taking any damage. Pretty self explanatory
2. MT
Tank's job first and foremost is to hold aggro and mitigate damage. Now for arguments sake we can say that being "efficient" is mitigating tankbusters only so that you may survive and hold aggro.
We could also say that being "efficient" is maximizing DPS whilst surviving, whilst holding aggro.
Dropping grit means that you are increasing damage taken as well as damage done. You would need to personally calculate (also calculate healer ability) whether or not the trade off is worth the drop in grit or not.
Be warned that dropping grit in 3.2 also means a much lower output of enmity thanks to changes made with enmity calculations and such.

All the above info was generally for raiding. In casual content, you can experiment when to drop grit in regards to aggro/damage taken/damage done calculations and then take that experience into raiding if you wish.