Depends on whether they are the O-ring seals or metal to metal leaks. Now most of your OEM fittings are O-rings or something. As long as the O-rings are in good shape, they only need to be tightened up enough to make sure they don't vibrate loose.
The auto manufactures have largely abandoned flare fittings because of persistent problems with leakage. I know, I know, you can keep them from leaking. Good for you, however, I have seen many filthy messes where oil accumulates from small seepages out of flare fittings. Be very careful not to scratch or mar the mating surfaces. Tighten them up fairly tight, you should be able to feel the metal springing a little long before your strip it. Check them after you start the engine, and tighten them up a little more if they leak. If you don't have the special wrenches made to fit over the line and contact about 5 sides of the hex, buy them, and not cheap ones. Even the best open end wrench may round a brass flare nut before tightening it enough. Don't use anything on the threads. The seal is at the flare. I don't know what it is about brake lines, but I seldom see any leaks with their flare fittings.
The tapered pipe threads are different. There the seal is dependent on good pipe dope. I like Loctite PST in the tube better than Teflon. Read and follow directions with either.