Never heard of anyone pinning themselves...it doesn't have to be a big fancy ceremony-just someoneto read the orders of promotion and someone to pin the rank on. I am posting a copy of the reg. Promotions and Reductions for you...it's been a while since I've studied it, but I don't remember anything like that.
http://armypubs.army.mil/epubs/pdf/r600_8_19.pdf
Edit:
I went ahead and read it anyway and I don't see anything about not having a ceremony, so I'm really not sure.
1–23. Promotion ceremonies and certificates
a. Promotion ceremonies will be held on the effective date of promotion. Early promotion ceremonies may be held
when the effective date of promotion is on a nonduty day.
b. The promotion certificate is not the official instrument for promotion. The promotion order will be used as the
source for grade, effective date, and DOR for all record and pay purposes.
c. A Soldier who is reduced one or more grades will receive the appropriate certificate when again promoted.
d. Promotion certificates for Soldiers promoted to SPC and above will be prepared by the unit or battalion (BN)
human resources (HR) for signature by the SGT/SSG promotion authority. Any higher level CDR may direct that
signature authority be held at his or her level, but the certificate will still be prepared by the unit or BN HR.