Setting TypeGuessRows will help if the JET provider is used, it doesn't really matter if it is the Excel source or a script task using OLE DB.

There is an equivalent setting for the ACE provider, you can find more details in the following thread:

http://social.msdn.microsoft.com/Forums/en/adodotnetdataproviders/thread/40f62ac7-7a14-44a7-9e38-115fde4f0e66