90460 & 90461 vaccine administration codes

Our pediatric practice bills many claims with mulitple vaccines. So far, we have billed the new (for 2011) vaccine administration codes, 90460 & 90461, referencing the distinct diagnoses for each vaccine without adding .76 modifiers. We have been pleasantly surprised at how few have been denied. During follow-up on a few of those denied as duplicates, insurance company reps have suggested we use modifier .76. Given our success so far, we hesitate to make this change and perhaps experience more problems than we have had so far. Any suggestions? How are other practices billing these claims and what are their results?