I would doubt you will damage your speakers using either method. However, what you would gain, or limit yourself by doing so is another matter.
There was a post on the board with a URL that provided direction on using Cat 5 cable for speaker wire. In looking at this website, I could not help but think of how much time a person would have to invest. The cable is not designed for this purpose, but inventive minds, and spirit will prevail!
Technically speaking, the Cat 5 cable would be of less size than a 12 gauge cable. This would increase different components of the cables load as presented to the amplifier and speaker.
My thoughts are to purchase a good cable, and spend the time in other ways.