Do all ATmega32u4 QFN ground pads need to be connected?
Having multiple GND pins serves several purposes:


Lower ground inductance. In this case, the pins will be internally connected, and not connecting one to GND will worsen performance, but it could still work. Maybe. I mean, probably. I mean, I could stop my bicycle when one brake cable snapped, so why do I need two brakes, eh?
Isolate AGND from DGND. In this case, they are not connected internally, and omitting the GND via from AGND means your ADC will not work at all, or just burn.


Are vias that expensive? Last time I checked they weren't, really...