Speaker Cables - Optimal Length

Greetings. This site rocks! I have learned so much here. I am relatively new to this hobby and this is my first post...

My current setup is...

Mcintosh MC275 Gordon Gow Commemorative Edition
McIntosh C22 Reissue
Rega Planet CD Player - Original
JBL L100T - Paid $100. Great Value for now but will upgrade

I recently bought some Kimber 8TC. I decided to buy two 15 foot cables although I currently only need about 8 feet per speaker because I wanted the flexibility of moving and rearranging my room. After buying the cables, I noticed that most speaker cables are generally 8 feet. My questions are...

1. Why are speaker cables generally 8 feet? Is it because of signal degradation issues?

2. Are my Kimber 8TC 15 footers too long?

3. I do not have them terminated yet. The instructions state that bare wire is preferred, and/or spades or banana plugs. Is it advisable to keep them bare?

4. Anyone using a similiar setup to mine and can give me any cool feedback on how they do things (not necessarily just cable related) would be appreciated.

Thanks!

Dan.