The notes E#, Fb, Cb and B# only exist in theory and sheetmusic as far as the guitar goes. E# and Fb and fingered in the same fret; Cb and B# are fingered at the same fret.
My talented fretless instrument playing friends tell me that when playing classical music in an orchestra, they differ slightly in pitch depending on which key they are in and whether ascending or descending a scale, but just thinking about that makes my head spin.
Peace,
Rico