interesting question . I have my own personal views on this mainline/hooklength choice . I try and use a mainline straight through to the hook . My theory being that if i'm using 5lb mainline and a 3lb hooklink , the 3lb hooklink is the weakest link in the set-up . Now , you have a knot at the hook and a knot joining the hooklink to the mainline....and i think i'm right in believing that a knot is a weak point , so this gives two possible weakspots . So , if i was to run 3lb mainline , straight through to the hook , i only have one knot and therefore only one weak spot .
Now , i'm not sure of the true scientifics behind it but from my own experiences , i have 10 times more confidence in a straight through mainline than a mainline/hooklink set-up. This is displayed in my attitudes when i'm playing fish which i tend to play a lot 'harder' with straight through mainline. Maybe its a confidence thing but its what i've found best.
Last season i hooked and landed a 14lb mirror on 2.6lb mainline , straight through and not once did i feel i was not in control.
TBH though , if i was fishing specifically for carp or tench on the float , i would tend to use 5-6lb mainline as on my club lake , this is normally suffecient.