Staining Deck Before Install?

I'm getting ready to replace the decking portion of my deck (the structure itself is fine). Recently moved into a new house and the prior owners never treated the boards. They're beyond repair at this point.

While I'm at it, I am planning on replacing the ballisters/top rails, etc. The deck is on the second story, so accessing the outside of the ballisters to stain them after installation won't be ideal.

Are there any issues with treating and staining the ballisters prior to installation, as opposed to after? Anything else I should take into consideration as I move forward?

Thanks in advance for any help!