In a DWI case, officer first observes "bad driving" in a neighboring county and makes the stop just over the county line in our county. I suspect defense may raise some issue about whether the offense happened in our county. I'm covered by CCP 13.04, but wonder if a jury instruction is proper?

I rather doubt the defense will be arguing the offense occurred, but only in the adjoining county. Presumably you can prove that the vehicle was operated in Cherokee County and that any state of intoxication was not suddenly cured by crossing the county line. You should have no need for a "which side of the line" instruction.

If the defense raises venue as an issue, then the judge should instruct them of the relevant law like in any other issue.

Venue instructions are proper. In theory, we could ask for them in every charge, we just usually don't need them. I agree the Judge could (and should) instruct them in the charge of the relevant law, just like any other issue.