The biggest issue I've seen with reports is the merging of cells. This usually happens if I have multiple tablixes on the same page that would export to being in the same tab in a spread sheet. The only way I've found to get around that is to have each tablix on its own tab, that is, page break between tablixes.

Another trick to remove "hidden" or extra rows is to be sure to line up multiple items right next to each other, in other words use the tools to have tablix2 begin immediately below tablix 1 by editing Location values.

If your users are most interested in doing pivot tables of the data exported to excel, I recommend making sure the raw data formats correctly in CSV format and then encouraging users to export to CSV output files where they can take the raw data and mush it up to their heart's content.