I think what you're looking to do is called "tinning" the speaker cable. To do this, you would heat the the speaker cable, and apply solder to the cable itself and not onto the soldering iron. This should be pretty easy to do if the soldering iron tip is properly cleaned and has a thin layer of solder over it to help heat transfer PRIOR to heating the speaker cable.
I guess since you're going to stick it in a spring loaded input, it'll be okay, but otherwise (say you wanted to use these with binding posts) I would suggest you not to tin the end of the speaker cable. Even though the layer of solder helps prevent against corrosion, it makes the ends of the speaker cables very STIFF and this makes using the speaker cables a pain in the butt. I've done this before, and in retrospect, I think I would have preferred to leave it bare or go to Radio shack and get some cheap spades. Hope this helps...
Sunny.