I've used a lot of different wicking mediums. I've found the taste provided by sterile rolled cotton and organic cotton balls to be clean, without unwanted flavors, and without muting any flavor notes. I also really enjoy German silica, Ekowool, and XC-116. Overall, the difference in flavor between all is subtle and they are all great at representing flavor.
As has been mentioned, the key to cotton is that less is more. Very, very little cotton is needed, as it expands when wet, and holds a lot of juice. Wash hands before rolling a cotton wick to avoid oils in fingers effecting flavor, but if it does it goes away fairly quick. I haven't found any need to boil the CVS sterile rolled or organic cotton balls I've used. Some do, and I've heard mixed results, but many find it perfectly fine without boiling.