The nozzle needs to give way a bit in the event that the PCB you are placing is warped to prevent breaking components.  There are two trains of thought on this.  You can either implement the spring in every nozzle, or implement it in the nozzle holder.

I decided to use luer lock syringe needles for my nozzles as they are a precisely machined and commonly available standard.  The luer lock itself at least is usually precisely machined.  It seems to be hard to find them with the needles actually centered with no runout, but I'll deal with that later.  So I implemented the required springy-ness by spring loading the holder and allowing the hypodermic tubing which is the shaft to slide in the bearings.

The spring loading also has the nice side effect of preloading the bearings for reduced runout.

It also gave me a place to put a Z limit switch which can then be used for leveling and to detect nozzles of varying length.