I would say off the top of my head that it's not a bad idea to prevent two users from editing the same file at the same time. Two users can open the same file of course, on the network, in OpenOffice.org; just not edit it.
You might consider breaking the spreadsheet into two or more, then creating one master spreadsheet and using Edit > Paste Special > Link, to bring them together. Linking between spreadsheets usually works pretty well. Then one user can get at one of the smaller spreadsheets and another user can get at the other one.
This was first published in March 2006