Violins and family(typically) tune their instruments with 3/2 just fifths. You get the A (440) from the oboe and tune the rest of your strings with perfect just fifths. That means sometimes the cellos' C strings will be noticably too low in some circumstances so you'll see them finger an "open C" just above the nut to make it sound right.
It's 2% of a semitone off by my understanding. I thought I had pretty good ears but I really doubt I could pick that. Open strings do often stick out in general on string instruments though, for a combination of reasons, lack of vibrato and ability to micro-adjust tuning presumably being the main ones (but even the tone is different, I assume based on the difference between having one end fixed by a soft fleshy substance vs the wooden nut).
Except you very very often don't get an A=440, since a lot of orchestras don't tune to that pitch and early-music orchestras are a full half-tone below that, etc.
