I always replaced tires and tubes together. For the additional $30 for a
new tube, I always thought it was false economy to reuse them. As some
have acknowledged, the tubes stretch and may fold upon reuse. I usually
see cracking around the valve stem on a used tube. Perhaps reuse would
be an option if we flew 250 hours a year and wore out tires sooner. But
we usually have tires for 4 years or so. By then, I usually see the
valve stem starting to crack and the sidewalls cracking on the tires.

It seems like cheap insurance. But, others swear by reusing tubes. This
is a pretty good case of how different people will accept differing
degrees of maintenance. Absent any Advisory Circular or maintenance
manual prohibition against reusing tubes, it is "legal" to do so.

Nitrogen in the tires? I don't even use nitrogen in the struts.

Good Luck,
Mike