Depends... if it's just a generic Carrotus/Diamondus/whatever sky, it's not too noticable. If it's something more elaborate, like in Corrupted Sanctuary, or if it fades to some freaky color and for some unknown reason I am playing in 16-bit, then I pay more attention. It basically depends on how unusual it is.
(People don't do nearly enough with the ability to add 3D planes to their levels.)