The content of your wedding invitation depends on the choices of the engaged couple, and on who will be hosting (ie. Paying for) the wedding. If you and your betrothed are financing your own wedding, parents can still be mentioned although they will not be doing the inviting. It comes down to your personal preferences, yet again, when deciding whether and how to include divorced and/or step-parents.
If your wedding is to be very formal, make sure your wording reflects that. Spell out the dates and times. Also, do not include an R.S.V.P. This should come on a separate card.