IUDs got a bad rap in the past because of the Dalkon sheild - this old IUD had a high risk of pelvic infection, and it caused many people to become sterile. It had a multifilament string that allowed bacteria to climb up from the vagina into the sterile uterus.
The current IUDs have a monofilament tail that does not increase the risk of ascending infection. Cultures for gonorrhea and chlamydia should be done prior to placing an IUD, and IUD users should be monogamous (one sexual partner). More partners increases the risk of upper pelvic infection.
If infection is going to happen with IUD placement, it usually does so within the first 21 days after insertion. It is up to the discretion of the individual physician whether to give antibiotics after insertion.