Nobody I know would be daft enough to switch the APU off on a -300 down route with temperatures higher than low twenties.
If you've got ground conditioning, then of course you'd use it and not the APU, but if not, the APU stays running. Even then, the amount of cooling is inadequate but it's better than nothing.
I can't recall seeing an SOP stating that we must shut down the APU on every turnaround, and even if there was, common sense should prevail.