Let's say you have a wheel that has 0 lateral runout, or close to it. At what point would you say that a spoke wrench should be taken to it. How many tenths of a mm are we talking? I have a wheel that is about 2.5/10ths of a mm out.

.25mm. You are fine.

When mine get to about 2mm I'll bust out the truing stand.

That, or when I can't get my brakes adjusted the way I want, but that's around 2mm as well, depending on the brakes.

1mm each way maximum for new wheels. but I get them within 0.25mm each way.
there's also roundness and spoke tension to consider.

out of the three, equal spoke tension is most important.
it's totally up to the person working on the wheel if they want more attention to tension or trueness/roundness

It depends on how anal I'm feeling. If it's a hair out of true, it takes too much effort to get it 'just right'.

If it doesn't affect braking, then I don't throw it on the stand.

This, and if getting it back under 1mm requires the tension to deviate more than 10%, it's time to consider a new rim and try to figure out what the heck I ran over.