Well, as with everything, it depends. If it's privately owned or only a small number of people fly it, I'd say it's no problem at all, as long as it's consistent. I would say it's still only a minor annoyance if it's flown by a lot of people. I used to be in a group with a Taylorcraft. We knew the replacement tacho was duff but we also knew the airplane. We flew it by engine sound and airspeed and didn't worry too much about what the tacho read in absolute numbers, just what it normally read in climb and cruise and anything that was different from that. You could tell from the engine noise if it was getting close to redline, so that was never really an issue.
However, for a club aircraft that I'd be hiring at a fair market rate, I'd expect everything to work as advertised. And, if it were placarded, I'd expect it to be really obvious so no one would miss it.
Now, as to whether you were right to go or not, only you can decide that. As you're still around to tell the tale, I'd say you made a good decision. Remember that any guage can fail and that, as you get more hours in any particular aircraft (specific one, not just a type), then you'll find yourself flying it more by feel and sound than anything else. When I take off in the Pitts, I know what it should sound like, how it should move, I have no idea what the tacho says as I've never bothered to check. But I'll know instantly if something isn't right.
How's that for a definitive answer, eh?
Pitts2112