Which mode is most commonly used for voice communications on the 17- and 12-meter bands?

The 17-meter band (roughly 18 MHz) and the 12-meter band (roughly 24.9 MHz) are both well above the standard 9 MHz sideband crossover point. Therefore, voice communications on these bands conventionally use Upper Sideband (USB). These bands are part of the WARC-79 allocations, which were added to the amateur service later than classic bands like 20 or 40 meters. Despite being newer allocations, they follow the exact same historical convention for sideband selection as the rest of the high-frequency spectrum. The simplest way to remember the convention is: if the frequency is 10 MHz or higher, switch your radio to Upper Sideband. Since 17m and 12m are higher frequency bands, USB is always the correct choice.