Actually, in the old days, they didn't bother putting subfloor under
T&G flooring at all a lot of the time. The last house I lived in had
T&G YP upstairs & handhewn beams supporting it that were exposed. The
big problem we faced was dust sifting down. It was a mess, but solid.
All lengths ended on a joist, but since they were logs, you had 6 - 8".
It's been a while since I've done any flooring, but if it were me, I'd
make my decision based on the flooring. I'd want the flooring to all
end on a joist. If you're doing oak, you tend to get random lengths &
ending them on a joist is going to be impossible. You used to be able
to get Yellow Pine in full lengths & that would be possible to end on
joists & it's strong enough. Ditto with cherry.