You should use Xray to qualify your process. Frequency of Xray inspection after that will need to be determined by your process capability. If you use paste inspection process, once you verify that your profile and placement are good you in theory would never have to Xray one. We generally Xray the first couple of boards off the line and let it go after that. When we run CSP we periodically monitor with Xray. The most important things are making sure that you print paste well and repeatably along with fairly accurate placement, and a good reflow profile. You can also get a pretty good idea how well they are running by just looking at them visually with a microscope if all of the balls are shiny, collapsed, the same size, and you detect no or very minimal warpage you more than likely have a good attachment.