I don?t tune down. I just play in the key we?re playing in. Unless there?s a note below the range of the bass that?s absolutely integral to the song, I just play it up an octave. On rare occasions, I?ve detuned my E down as far as a D, but then I have to think really hard . If I have to play lower than that, I?ll consider grabbing my 6 string.
I once played in a blues band where the guitarist wanted to get as close to Stevie Ray Vaughn?s playing as he could (just don?t ask me to exactly clone the bass lines ? I don?t do tribute bands). He had 2 guitars. I really didn?t know (or care) _why_ he had 2 guitars, that?s his business. It turns out one of them was tuned down a half step. We were playing a frat gig where we had consumed our share of adult beverages. Prior to the last song, he breaks a string on his standard tuning guitar, and grabs the one that?s in Eb. I soon realize the difference between the 2 guitars, and make the shift down a half-step, but the song contained a unison riff that bottomed out on an open E. Oops. That was not an easy real-time edit after 4 hours of free beer.
I?ve run into a similar issue with folk guitarists who use capos frequently. Having never played guitar, the use of a capo strikes me as odd. The only reason I own one is for setups.
There?s a folk band I?ve been subbing with where I?ve had to ask, ?What key are you singing this in??, rather than ?What key are you playing this in?? because the guitarist insists he?s playing in G even though he?s got a capo on the third fret.
Our praise band guitarist also uses capos a lot. I give him flack about it, noting that jazz players figured out these chords a long time ago. We had one song with a modulation in the middle. He was using a capo. I asked him if he was going to stop in the middle of the song and reposition the capo.
That?s exactly what he did.