You are relying on browser functionality and the user's environment to ensure their data re-appears when they click back. You, as the web developer, can not assume everyone will have this experience.
Depends on how the forms are laid out. If they are like a "wizard" which gives the user the feeling of stepping through a series of forms, then a previous button would be a nice touch (and when they click "previous", it sends all the data collected so far to the particular page which makes up the "previous" page). It can be a bit daunting to do this, but a couple helper functions can make this go pretty nicely.
If you're doing error handling, and the user has to click back to fix their data, then instead of using that approach, redisplay the form with the user's entered data and show the errors above the form (or if you're fancy, next to the form rows where the problems are at). In a situation like this, the user has no need to click back since all the data is right there.
