Welding; How much undercut?

Some of the welding manuals I have say that a certain amount of undercut is
inevitable. Certainly in examining the fuselages of some factory aircraft
I've seen some shockingly large errors like this in the finished welds. On
many of my practice wleds, particularly on tubing clusters, an otherwise
perfect weld has a tiny bit of undercut. I'm talking barely visible here,
only just able to catch the end of a rod, for instance. At a guess i'd say
no more than .02 inches, maybe a bit more. Is this an acceptable situation?
If anyone's interested, maybe I can post some pics of one on a wesite but I
don't think the undercut will show up on a pic, it's that little...