In my opinion, you should tune the suspension without LSD interference, once the suspension is sound, set the LSD to match the suspension.
When done any other way, the LSD can, and likely will mask some suspension quirks that would have been ironed out if the suspension was tuned before the LSD. The LSD can be used even in replacement of good suspension if you're really good at it and the car is 4WD, but the quirks will remain, because it's next to impossible to decipher LSD problems from suspension problems when both are in play.
LSD only has power when the engine does, whether accelerating or decelerating, so if you're not accelerating or decelerating hard enough to activate the LSD, it can't really help, but suspension is always on, always working.