Even if you don't mount it up on the top of the clamp (or anywhere else you can see) you should have an odometer somewhere on the bike.
I know with cb750s, certain things start to go at certain mileages so I know to watch out for them.
+1 on keeping the tach, but mostly because my ear thinks the bike is screaming readline when the reality is I am probably only at 5K rpm. One of the neater tricks I have seen is that guys running only tachs on the street will make a mark on the tach at what rpms 55mph is in top gear, that way they know where to be to drop to the legal limit should they run into a speed trap.