First thing I did was put a tach on my Touring, too. I rode a 4 speed Kawasaki for about 5 years without one, but always wished it had it.
I wanted a Rocket classic mainly for the 240 rear tire and the tach, but got a sweet deal on a 2010 R3T.
I don't get the tach-speedo gear thing yet, but I often discover I have an extra gear to use when over 70 MPH on this touring!!