Gill,
I agree with Buddy, ... If they're cheap, get 'em all.
I spool 3 different line wts for the same rod.
Also WF vs DT lines in the same wt can make a difference. I'll mix 'an match lines and rods for different conditions and lies.
When giving lessons, I'll also mix up combos quite a bit to find te right balance of flex, lenght, action and loading depending on the student's "personnality".
I'll typically underline, cause I like double hauling and load the rod in a kind of brutal fashion, ... but that's my way of doing things.
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Christopher Chin
Jonquiere Quebec