You really want them nice and tight so that they respond quickly. If they are loose the transmission from leg muscle twitch to ski slows, and the reaction of the ski slows. I only ski resort steeps and groomers, so no park, so maybe it is differently with park, but for steep terrain or racing or groomers or bumps you want your ski to respond to every precise movement you make. That requires tight boots. If you are skiing with loose boots in steep terrain or at high speeds you are sacrificing performance. You want a fit that is as tight as possible. I unbuckle my boots on the lift. I don't even buckle the bottom buckles when I ski. I have foam injected liners so the fit is very precise and I don't need to buckle the bottom buckles. You want your boot to fit like a glove, nice and snug. If you have points of pain go see your bootfitter for a punch out, or maybe you got a bad fitting job and the boots are not the proper boot for your foot. A good bootfitter makes all the difference. Get in the wrong boots and even if they are properly sized you will suffer and so will your performance. Boots are more important than skis when it comes to performance. Do yourself a favor and get a pro to size and fit you properly. If skiing is important to you get that fit right.
top 2 buckles should be kept tight, these are what provide all the performance in your boot, 2nd buckle down is the key buckle to pull your heel backward into the back of the boot.
Bottom 2 buckles can basically be left undone as they provide virtually no performance, if you are having to crank these down to feel secure in your boot there is a good chance you're in a boot that is too big for you (volume).