The bane of multistep node forms are validation errors. But Drupal 7 has a cure. Not nice but a cure.
#access = FALSE on form elements you dont want to see in the current step and then set up the Next button with
#limit_validation_errors set to the elements you are about to see. The Prev button should get an empty
#limit_validation_errors. Should field validation errors bother you, you can nuke those in