idle speed question
 

I have a 1999 Honda Civic LX Sedan M/T with 150k miles, and at operating temp it will idle in neutral around 450ish, I was wondering if that is too low? I downloaded a service manual off the internet and it says 670 +/- 50 at one spot, and 450 +/- 50 on another, so I'm not sure which it should be. The engine has never died and the rpms stay steady at idle, they don't jump around or anything. The car just seems to vibrate a little more, so that could that just be a certain resonance freq? or I've heard engine mounts being worn out.

My only concern is if it is idling too low. Engine mounts would be too much work to fix and I don't care to put the effor into that.