Using Octave Strings of Differing Gauges
There are a few players who use octave strings of differing gauges instead of exact pairs. For instance, the high-octave G strings could use a .020 gauge string paired up with a .018 gauge string. The reason behind this is that it may give a little different sound to the instrument. The differing gauges will resonate off of each other in a different manner than will strings of the exact same gauge, creating different tonal possibilities.
Using strings of differing gauges may result in changes in neck tension. For instance, if a larger gauge octave string is used, it will likely have more tension when tuned to the higher note. And a smaller diameter string might be a bit floppy when tuned to a note lower than usual.
A problem that may occur with using different gauge strings is that they will not intonate precisely unless the bass incorporates a 12-saddle bridge. But since many players slightly detune the two upper octave strings from each other anyway in order to get a fuller, chorusy sort of sound, an 8-saddle bridge may work just fine.