Hmmm... Perhaps I'm being entirely redundant here, but:
When I knew I needed to save stuff in a spreadsheet compatible way, readable by all versions of excel, and maybe some odd ones people used for kicks, I saved into a simple .csv file.
When you say you need to export to excel, does this just mean something readable by excel, or does it, of necessity, have to be in the actual MS excel format?
If you're just saving it to a datafile for easy manipulation, as I've read it, I'd say go with the CSV format.
At least for starters, as you can then go and check the data you've saved, and be sure any bugs that arise are in your saving routine, not the excel formatting.
CSV format is also presented as an excel icon, I believe as standard, so you shouldn't have a problem with people who need to see icons..
Anyhow, that's just a thought,

Malk
*I lost my .sig. Do you have a spare?*