The advice the OP got from the LFS and here are both correct, as has been pointed out.
One thing I noticed after using "live" sand to seed my tank, you have to be extra cautious if doing a fish-in cycle and seeding the tank with a good dose of BB.
The reason being that with a normal cycle it proceeds through the various stages; ammo> nitrite>nitrate and each spikes respectively.
But in a BB seeded tank ALL levels will spike concurrently and produce a much more toxic environment. The cycle will complete faster, but you encounter worse water chemistry initially.