A few years back my band played in Standard, Drop D, D Standard and Drop C. We each had our own methods, lead guitarist just tuned up and down as necessary. The rhythm guitarist swapped between D standard and Drop C, then used a capo for the E standard and Drop D stuff, then i just went the complete lazy method and got a 5 string. If i didn't use a 5, i'd have gone for multiple basses for certain. It makes more sense, and it's easier. Means you've always got a spare too.
Liam