oh and, if you click on the advanced tab on the share worksheet page you can enable change tracking on the worksheet, which should let you roll back to a previous version of the sheet if there is any problems.
Also depending on what version of windows your servers are running here, but if it's server2003 or later than then volume shadowing is an excellent tool to protect against unintentional data loss (it's saved me a whole lot of work on several occasions, again speak to your IT department about it)