Actually, if you buy an app when it's on sale and then try to download it again after it goes off sale, it won't let you.

I once caught a an app that was temporarily free and got it in iTunes with my account it in one of my computers, expecting it to show up as an available download when I got back to the one I sync my phone with. It doesn't appear and I had to copy the .ipa file there.

I have no experience trying to redownload paid apps that are still the same price, but first thing I did was to add .ipa files to my daily mozy backup.