Here‘s the deal... even if you do show up looking like a slob, it‘s not going to automatically count you out of a career in the military... that‘s why they‘ve got basic training, to beat dress and deportment into your head...
The interviewer is going to be concentrating primarily on the answers to your questions. If you seem disinterested (I‘m sure it‘s happened) during the interview, or you seem like you just don‘t care, you‘re not getting in.
That being said, as somone else pointed out, shirt and tie for an NCM, full suit for an officer is a good bet...
Regarding showing up over dressed for an interview, prior to joining the reserves (Or more accurately, after joining the reserves, but prior to my actual training begining, which took a good six months, but well worth the wait), I was desperately looking for a job... I dropped into one retail store, and they said "No, sorry no openings", but I was polite and friendly, and out going with the manager anyway, and said "Oh well. Thank you anyway, if you have anything come up in the future, let me know". At this point I was wearing a dark polo shirt with a pair of casual dress pants, looks spiffy without looking over dressed.
Anyway, as I was about to walk out the door, he said "Wait, actually, we‘re doing a group interview for a couple of posistions tommorow morning... can you drop by?"
"What time"
"Well, it‘s a bit early I know, but 8:30"
"Not a problem, I‘ll see you tommorow morning."
Now, I didn‘t know what to wear, so I figured safe bet, shirt and tie. As it was, most other people showed up looking like slobs (Maybe 30 people for 6 posistions), with the exception of myself and two others. We were hired.