As you seem to know, C6 is the overall standard today and for a long time now, A6 is still popular as well and was a country standard decades back and finding favor again today. You can't go wrong with them. Just try a few different tunings until one 'speaks' to you, and then play the crap out of it every way possible before you mess with another.
I'll offer up my web sight and it's tutorial page (pg. 4) to help get you going in the correct manner, ala Jerry Byrd -
www.hwnsge.5u.com
I havn't taken the time to figure out what tuning these Japanese guy's are using, but you can do most anything on any tuning, so just find the one you dig and works for the style/s you prefer.
I use a glorified E Maj - G# E C# B G# E B E which gives you a majority of the old Hawaiian, and most of the Don Helms/country style. You get a nice full strum and partials, and allows for lot's of slanting options.
- Watanabe Naohiko www.youtube.com/watch?v=YIHvoDIZ92U&feature=related
