Adding LR in stages??

[JDigital]

Is there any pro's or con's (more my concern) to adding LR in stages (10-20lbs) at a time?

Will I experience multiple Ammonia or Nitrite spikes by adding it in stages? Or is it best to just buy all that you need in one go?

I ask because, I am a fairly picky guy when it comes to the LR I see in the shops and generally looking for a certain size/shape/etc piece.

Cheers
Josh