Manufacturers give quite a lot of leeway in expiry dates...which are just the date to which they guarantee the results if the film is stored in accordance with their instructions.
I stock up with new and fresh film (B&W or color) from time-to-time, keep it in the fridge or freezer, and not worry at all if I use it up within 2-3 year after expiry. But I'd be much more cautious in buying out-of-date film unless I really knew the storage history (perhaps being a little skeptical when an Ebay seller says it's been frozen since the day it was made...no offence meant to the many honest Ebayers, of course!). Not worth the risk to try to save a few $, when the real cost is spending time, money and effort in getting to photogenic locations.
Having said that, I have occasionally used really ancient film with some degrees of success, but only by way of interest and experimentation, never for important pictures.