Leaching - Is circulation in pipes needed?

Are you stripping the nickel off the tank? If so, I would at least run the pump for a brief period (5 minutes)to get the nitric in there, as there could be nickel in the pipes. If you are leaching a new tank prior to a bath makeup, just get the nitric in contact with any virgin plastic. Run the pump for a couple minutes, then shut it off for the night.

I can see that some readers believe you are talking about stripping an electroless nickel tank and its piping, whereas I felt you were trying to strip plated parts--so you might want to clarify.

But assuming you are talking about parts, I haven't done it, so I can't really say; but I have done somewhat similar things and I believe the answer is going to be empirical rather than scientific. I suspect you'll have to jiggle or agitate the parts but won't have to actually pipe them up and make solution flow through them.