I agree with the majority here that using the candidate title just doesn't sound right to begin with.
I find the whole thing baffling. Titles are meant to convey something important--like your rank in an organization (like the military), competence with certain skills, etc. I just don't see anything worthwhile being conveyed by "MD candidate" aside from being able to brag you got into medical school. Which is really great and something to be proud of! But I don't think there's really a place for it other than on a CV as "currently enrolled."
I don't get why people bother to list "M1, M2, etc." either for that matter. I'm sure it's there as a matter of pride/accomplishment, but to me it just comes across as bragging and always rubbed me the wrong way. I mean, anybody that actually cares about your med school year already knows what year you are, and all the others are just going to be confused.
On the other hand, people that include BA, BS in their signature (especially when it's not related to their job) confuse me as well. Unless it's a professional degree (B. Arch, etc.), it's just not really conveying much.