The advantages to melting are the thing just wont break if you bend and twist it about into whatever shape youre trying to get it. Its good for complex bending of metal into shapes you want. If you do choose to heat it, heat it to the point where you can bend it, but smoothly and with slight resistance. You'll probably want some needle nose pliars or something (dont wanna burn ure fingers

). The danger of heating is basically just overheating. When you do that, it "burns" the knife. Burning a knife ruins the temper of the steel. It will either become too brittle or too bendy or what have you.
Cold bending is ok for bending up to around 90 degrees. More than that and the metal will get weak and snap. A vice and pliars are usually helpful. Maybe a small hammer to help. Cold bending will be sufficient for most tension wrench type bending youll have to do.
If you do choose to heat it, butane lighters are perfect for the job. Just make sure its a torch lighter (one that makes a blue flame with that rapid decompression sound). They burn hot and heat fast.