Arduino Micro Power Questions
If I'm powering off of just USB, then I can connect +5V to VUSB. Correct?


If you are powering off USB leave everything alone. See the schematic:



The blue line added by me shows what is going on. The VBUS signal from the USB connector goes through a 500 mA resettable fuse (to protect the USB port from overloads). Then it goes to the P-channel MOSFET (FDN340P) which makes a decision about whether to switch on and connect VUSB to +5V signal. It does so if VIN is low, making the MOSFET conduct. If VIN is high (which it would be if you had power into the VIN part of the board) then it disconnects the USB from the +5V line.




  If I'm not using +3.3V, should I connect it to GND (instead of leaving it floating)?


Definitely not. The 3.3V line is an output from the 3V3 voltage regulator. To connect it to ground would heat it up and destroy it.




  If I can provide a steady 5 V source (connected to +5V), then I don't need to connect anything to VIN. Correct? If that's right, should I connect it (VIN) to GND, instead of leaving it floating?


No, VIN is already connected to ground via a 10k resistor (see schematic above) and thus it is not floating.