The Amazon Fire TV Cube is the first Fire TV device in a good while that isn't meant to be tucked behind your television, out of the way. (But it will need to be relatively close to your TV, since it's going to be plugged into your TV.) That's obvious because it's (a) not a stick or pendant or dongle, and (b) because it's also an Amazon Echo device.
That last point means that it's going to need to be able to hear your voice. And for that to happen, it means you're basically going to need to be able to see the Fire TV Cube from where you plan on watching TV the most. Far-field microphones are great, but they won't work so well if the Fire TV Cube is hidden inside a cabinet, right?
Amazon suggests putting the Fire TV Cube at least a foot or two away from your speakers. (That could get a little weird if you only have a sound bar and no side tables or anything.) You'll also want the Amazon logo to be facing you — so the mics will be able to hear you better.
Everyone's setup will be different, of course.
And Amazon should be commended for the way it's communicated where your Fire TV Cube should sit. It's detailed on the protective plastic that you'll have to take off to use the thing in the first place, and it's also in the included quick-start guide. (You read through that, right?) Plus it's clearly within sight when you're watching the intro videos.
One other consideration though, is if you're going to need to use the infrared extender. This is a little wire connected to an IR "blaster" that you can then tuck away inside a cabinet to control any devices you've got shoved in there. (It's one of those things that's insanely useful if you need it, and utterly worthless if you don't. But the point is it's an option, and it may affect where you put your Fire TV Cube.)
To use the IR extender, just plug it into your Amazon Fire TV Cube, then place the other end near the device that you want to control.
And if for some reason you want to use a third-party IR extender (maybe it's longer, maybe it's prettier, whatever) Amazon says that should work just fine.
