I've been through the guts of hundreds of computers, both desktops and laptops, and have re/built a number of them, often working on them with only a frame or even no case at all-- just parts on a bench. I've never seen any difference. I also play electric guitar, sensitive to radio and electronic interference, and can attest a pc case makes no difference.
My favorite mini-server I built for low power-consumption from the guts of an old HP g-series laptop, bolted to a sheet of plexi-glass. All the foil shielding makes no difference. It's run for years without any errors caused by electronic interference, transmitters, cosmic rays, or solar flares-- though that last bit did measurably affect wifi communications between other devices and the AP's, but those were on another network segment and physically seperated by a router and 5 to 100 feet of space, so obvi would not have been in the case anyway.
I actually tested this with various transmitters and power-supplies brought in close proximity. No perceptable difference, and nothing in the error logs to suggest it was affected. A guitar pickup likewise is affected the same by PC's with or without a case. Kind of a cool, practical test for outgoing interference as you can actually hear it. Also necessary if your a guitar player. Grounding the PC also seems to make no difference to the PC in terms of function, though possibly in terms of safety in extreme cases, but you don't need a box for that.
If it is a concern, and you like wood cases, you can always line it with foil, or foil tape. A sheet of Reynold's behind your AP rounded a bit like a dish works wonders for reflecting or focusing your wifi signal, just like the reflector in a mini-maglite. You can test it with a wifi analyzer app on your phone, so it will definitely do a lot to keep interference in or out of your box. But like I said, I have tested it and see no practical difference.
For an easy real example of interference, try having a conversation, listening to Pandora, downloading something etc. on your cell-phone within 3 to 5 feet of your average microwave oven. If it's not one of those cheap dinky one's, you will notice significant chop, or even total signal jamming. If that's considered safe, I think I'm ok without a metal box. My naked server sat on a shelf on my desk less than two feet from my skull. I've never noticed any difference there either. More worried about my laptop in my, well, lap.
Now... where did I put my tinfoil hat? ;)