Everyone has questions about wireless networking — and that’s totally OK. There’s plenty you might not know about wifi, even if you’ve successfully set up your router and are sitting there, right now, browsing Lifehacker on your laptop, desktop, smartphone, or tablet.
Were I you, I’d stick my wireless printer on 2.4GHz, but you’d also be fine going with 5GHz if you really want. I don’t know what printer you have, but I’m going to guess that all wireless printers can connect to 2.4GHz wifi networks, and there are probably some that cannot connect to 5GHz networks. That’s the only reason to pick one over the other, to be honest.
Since your printer is sitting so close to your router, either band should be speedy enough for your printer to use. (I presume you aren’t sending over huge printing jobs that suck up a lot of bandwidth; if you are, or if you have a wireless-ac printer and want to use it to its fullest potential, you’ll want to go 5GHz.)
Your connected devices should all be able to see each other whether you’re using 2.4GHz or 5GHz — no issue. There are some instances where they might not, but this isn’t typically present in a default router setup. For example, if you’re running a guest network, devices connected to it might not be able to see anything connected to your primary network (either by default, or because of a setting you’ve toggled when setting up the guest network).
Your router might also come with some kind of “access point isolation” feature — similar deal. If you’ve enabled that, and you probably shouldn’t, then connected devices shouldn’t be able to see any other devices on your network. They’ll be able to access the Internet, but that’s about it.
Otherwise, the decision to connect to 2.4GHz or 5GHz is a matter of range and speed. While it’s not a guaranteed rule, I’ve found that 2.4GHz lets you squeak out a bit more range — albeit slower speeds — than 5GHz, but if you’re in range of both, go 5GHz for the fastest possible connection. And if your devices are simple and don’t need that much speed, throw ‘em on 2.4GHz and keep your 5GHz band free for your bandwidth-hungry devices.