I don't see any need to keep iTunes Store receipts unless you want to have them for proof of purchase in case you need to file an insurance claim if your computer ever gets stolen. You can get the same information from your account information in the iTunes Store if you don't have the receipt, though in a bit less convenient form.
You can always copy the receipt emails to a text file and save them on your hard drive if you want to keep them but get them out of gmail (if gmail doesn't offer a direct way to save an email to text).