My 2 cents is not about the copper pipe/ stop valve compression fitting (I do believe, as HJ said that you don't have to be too delicate when you tighten this nut) but at the other end... flexible stainless steel supply hose to faucet connection. I think I read on this forum where this nut should not be tightened to much otherwise the rubber gasket could get mangled and ruin the integrity of the seal. Well, a few months ago I installed a new lavatory faucet along with a new 1/4 turn stop valve and new supply hose. I tightened the subject nut, hand tight and about 3/4 turn more with a 10" wrench. Turned on the water and viola, no leaks. Monitored it for a couple of days and I thought I was home free. The night after I last looked at it, the hot water connection started leaking and in the morning when I went to use the bathroom, a plastic tub (holds about 2 gals.) under the stop valves/faucet connections (this is my fail-safe strategy) had overflowed and dumped another couple of quarts of water onto the cabinet base and on the bathroom floor!! I was P.O. to say the least! My critique is that when I used the hot water that night to the extent that it heated up this connection, there was some realignment/shifting in the mating and it started to leak. In my initial test, I neglected to run the hot water long enough (it takes a couple of minutes for the hot water to reach this faucet) to heat up the connection and so it was not a really good test! Live and learn... Oh yeah, I tightened the nut about 1 and 1/2 turns more and so far so good. Plumbing for the diy'er is tough!!