I know this is 10 years old, but a quick update is in order.   The solution to having Excel have any frozen panes, is somehow involved in the use of a Matrix, and not a tablix.  While you can have the display in SSRS have fixed header rows, that just doesn't make it into Excel when exported.   I've tried mightily with every solution out there on the web today, and all for naught.   A relatively recent assignment of mine required a matrix with a column group, and that actually worked.  Nothing I've ever tried either before or after has ever worked in Excel, and I'm talking for at least the past 10 years...   fyi in case you are banging your head against a wall like I did today.