From what I understand, only voluntary inquiries from the last 12 months are listed on your report. So technically, if you request a report once a year, you should be fine as single inquiries alone do not have an effect on your score. However, if you request a report shortly after applying for a loan (house, car, student) or credit card, then it could damage your score because now you have multiple inquiries within a 12 month span.
Also, I've heard that if you are shopping around for the same type of loan (not credit card), like a house or a car, you will not be penalized for multiple inquiries of the same type.
EDIT: Ah, missed one of your questions. You've heard right, you can be penalized for requesting mulitple credit reports per year. The safe bet is once every 18 months or so to ensure the last inquiry has cleared from your credit history. Sometimes the agencies are slow. :|