BT's systems can't distinguish between a reboot/power down or loss of synch I was told.
The best practice would be to power off the modem, thus allowing it to send a "dying gasp" to the DSLAM/MSAN. Then remove the WAN link cable and power the modem back up for any configuration adjustments.Whether one rigorously follows the above will be directly related to one's experiences!
BT's DLM system does NOT take a blind bit of notice of dying gasps - it has its own system to detect what it calls Unforced retrains. .....Whether BT's system of detecting unforced retrains is more reliable is up for debate, because it only assumes unforced if the EU has a full 15min bin with 0 uptime.