Plumbing the Hard Way - Threading on Exisiting Risers

Am I asking for a whole heap of trouble making threads on existing risers.
I've always done threading on pipe on a tripod, fully secured and fixed.

I'm in the planning stages, and what I am thinking is that once I have the walls open, I would cut the risers (brass risers) at the appropriate height where I plan to establish the shutoffs, secure the pipe.....and just tap the threads in it's current place (assuming there is clearance between the H/C risers) to accomplish this. Too risky or possibly making something downstream leak ?

Option B would just to establish wherever there is a T in the existing run, cap off one End, put a shutoff there and start the routing of the feeds from there.

Either way will work, although finding a thread to start from will be a lot less work.