Boeing operator here (not on the B744)
Before starting the taxi-out we put the transponder in XPNDR, which basically sends signals to the ADS-B/Ground radar system. After vacating the runway, upon landing, we do the same. This mode shows a TCAS OFF message on my EHSI.
One airport in particular in our route network ask us not to do it, keep it STBDY, as their ground radar is an older generation and it would clutter the screens (or so I was told by an ATC chap).
When entering the runway, for departure, we select TA/RA, until vacating on arrival.
Now, en route, when asked to chance codes, most of the colleagues put it on STDBY, dial in the new code (or type it in, depending on transponder control panel type) and then back to TA/RA. I personally find this unnecessary and change the codes without selecting STDBY first.