You're right about being able to open TIFFs multiple times and not lose quality. TIFF is what's called a "lossless" compression format, which means it does not discard any of the original image data. JPEGs will throw away a bunch of extra data that viewers are unlikely to notice, in order to make the image size smaller. This means that if you try to go back and edit a JPEG image, the image won't look as good as if you had edited a TIFF (although depending on how much you do to it, this might or might not be perceptible). The downside to TIFFs is that they take up a LOT of disk space compared to JPEGs. So it's really up to you whether you want to go for tip-top image quality, or if you want to conserve your hard drive space.