At top of climb reduce MP first to about cruise value, set RPM to cruise, then re-adjust MP to more accurate cruise value (it will have changed with the RPM reduction). At top of descent, the common practice is to leave RPM at cruise and reduce MP. On final approach, increase RPM to maximum (in case of a go-around). Experienced pilots will tend to do this fairly late on approach to reduce noise and drag, and with some geared engines to minimise time with reverse loading on the reduction gears.
Having said all that, I am not familiar with the Rotax, other than I understand that it runs a reduction gearbox which may have some unique loading or RPM limitations. Your instructor may have a technical reason for going to max RPM prior to descent. Ask around.
Also, is the Rotax set-up a true constant speed unit or just an in-flight adjustable pitch prop? Not that it would make much difference to the above-described technique unless there are specific RPM bands to avoid.