Google gives you the option to have your ads include the Advertise On this Site line -- I think it looks better without but am not sure if removing the line affects the earnings?
I've set this not to show. I think it takes a day or two for the change to take effect. I can't say I've noticed a big difference. Some say that without the "Advertise on this site" line, the Adsense looks less obviously like advertisements, so you may get a slightly higher clickthrough. Unfortunately you can't experiment with this on a per-channel basis: either all your ads have it, or all of them don't.
In theory it should increase your CTR as opting out of the program would remove the text which would help with blending. Whether or not this is actually the case is another thing. Personally I have opted out and will continue to do so until I am happy with my earnings.
you have no option. and its account-wide. its recommended for high traffic sites to keep it to assure advertisers can jump in easily.