I have been using top water frogs and one of my friends said your supost to cut the strings hanging from the back half way off is this true.

I trim a half inch off only one side to unbalance the frog and make it easier to walk. But you do not half to do this.

ive trimmed up to an inch off on some. I tend to trim a little off the smaller frogs, but I like to leave the bigger frogs with full legs. Shorter legs helps create more of a walk the dog action. Some people like it some people prefer chugging it. its up to you.

Fish it first before you modify it. Once you start removing things...you can't undo it so learn the bait before you modify it. That goes with a lot of things, not just frogs. Most frogs I used, I don't even need to modify it. Once in a great while i'll trim the legs but I just don't see it as necessary as people make it out to be. 

I trim mine sometimes just so that I can have a smaller profile.