Generally, the higher number is better, obviously. Whenever I'm saving as a JPEG for my own use locally, it's gonna be set at 12. But for web use, you almost always want to compress the file much smaller than that to reduce loading time. However, I'd use the File>Save for Web option as it gives you much more control. The only time I use the standard JPEG file dialog with anything lower than 12 is if I'm in a hurry and just need to save a comp image to email someone, for example. Also, Save For Web doesn't get along with really large dimensioned files that well.