If you remove the pinion flange, you might as well replace the seal, but if it's high mileage, it may have a groove worn in to the pinion flange sealing surface.
As for torquing it back to spec, when I've done mine in the past, I always marked the nut and the threads on the pinion shaft, and was sure to re-tighten it to that same spot. The crush collar takes a TON of force to tighten it past the original point, so when you tighten it back to the spot it was at originally it will pretty much stop right there unless you're using a 5 or 6 foot bar to tighten it.
GB