For line to leader, a loop to loop connection is handy for changing leaders. Theoretically the strength of this connection is of minor importance since your tippet would break before this junction does. That is why there are many methods of making the loops - they all work almost all the time. On fly line, I have tried whipped loops, slip-on loops, nail and needle knotted mono, etc. There is even a zap-a-gap no-knot join. The end of my flyline gets beat up from flailing away - this can lead to failure but it does not happen very often. Out of curiousity I ran a strength test with a nail-knot versus a whipped loop. The whipped loop was by far stronger - the nail-knot pulled off the line first every time. So, I use a whipped knot. On the leader end, I only use a perfection loop because the knot is relatively small. Just my experience, others will have different preferences.