Make sure you are using the exact manual for your bike, year and model. Not all XB wiring is the same.
If the rest of your cluster works OK while having the erratic tach needle, your grounds to the dash are fine. NO need to play with anything or add anything or clean anything. K.I.S.S. If you go messing with stuff now and cause a problem, you won't know how to back-track and fix it.
FIX it first, then modify it.
Your problem is in either the tach signal to the dash (fix the signal), or inside the dash (replace the dash). Checking the signal at the dash plug will tell you which one it is.