To keep a detailed history of the approval processes, It could be necessary to keep the workflow history after 60 days (predefined cleanup in MOSS 2007).

See the following article : http://technet.microsoft.com/en-us/library/cc298800.aspx

The main point of attention is the fact that "workflow history" is a Sharepoint List and could it decrease MOSS's site performance if the list exceeds 2000 items.