VFD tachometer sanity check

I'm thinking of buying a variable frequency drive for my Bridgeport.

The 10 Volts off the analog contacts of the VFD can be set to be
proportional to the frequency.

If the RPM is linear to the frequency (after slip has been taken
into account), and if the voltage is linear to the frequency, then
shouldn't the voltage be linear to the speed?

If this is the case (the real question I'm asking), once the setup
is calibrated, this should give the ability to have an accurate
electronic analog tachometer on the control pendant I'm designing
without resorting to an additional optical encoder.

I figure a tachometer would be useful for "manual" machining and
sanity checks on CNC milling.

Any thoughts, comments or suggestions would be appreciated.

Jeff