The best way to NOT round off the nuts is to use the correct type of spanner……… “which is that?” I hear you ask
The correct spanner for brake and fuel lines is a flare nut spanner.
Below are some pix of mine
There are both metric and imperial sizes in the pic above, you’ll notice that they grip around the nut and as such tend not to round the nut off like a normal open end spanner. The fancy ones in the tool roll have a ratchet action.
These are of a crows foot design which are handy if you don’t have the swinging room. They are 3/8” square drive, so whack on you socket extensions n’ ratchet and away ya go!