You should not have any problems viewing and modifying documents created with an older version of Office on a newer version of the same program; however, documents created in Office 2007 will need an add-on to be viewed/edited with an earlier version of Word/Excel/Whatever App.
When you open a spreadsheet created with older release of Excel in a newer release of Excel, you'll receive a prompt telling you that formulas will be updated when you attempt to save any changes, though.