That's cool.
I know what you mean about easily (or not) transposing by watching where they fret...!!!
I play both guitar and keys...I can get my head around that stuff most times.
The times I tune down is mostly when I don't like where the chords are falling on the fretboard, and I want that thicker sound.
Usually I'm only tuning down a 1/2 step...occasionally a whole step...rarely 2 steps.
With drop C you're only tuning down the low E string two whole steps.The rest are only just one step with the low E string tuned to the D string,which of course is now the "C string"
I use drop D flat and drop C tunings.It doesn't screw the action up as much as you'd think but you gotta set it up properly.
I havn't recorded my bass yet with drop C so it may get a bit floppy.When i bought the bass from my old drummer it was tuned to B and it was real floppy.
I had to reset the action but i havn't checked the neck to see if it needs a truss rod adjustment.I should do that real soon cause i plan on doing some bass recording once i get some of those felt picks.