It depends where you are getting the rock, and whether or not it is fully cured.
If it is shipping, you for sure will experience some die off and this will result in ammonia in the tank, which would not be good if you already have fish.
If you are getting it from a LFS
it really depends on how well they cure their live rock.
Personally, I think it would be a lot better to get your liverock, make sure the tank is good to go and fully cycled after the liverock has had time in the tank, and then add fish.
If you add fish first, you are taking risks that you really don't need to take.