I asked Garmin Tech Support the same question about my Garmin GPS35PC.
Quote:
The unit is designed for continuous operation, but you should also leave some provision to cycle power. Operation outside the rated temperature range or outside the rated power supply range (including undervoltage) can cause the unit to "freeze up". It's not common for a unit to freeze up, but cycling power to the unit will reset the processor and it will recover.
I leave mine on 24/7 but now I interupt the power every couple of weeks.