When you’re starting out with a teacher, you are probably told that the fretboard is a grid, but this is only half true. The fretboard is both a grid and not a grid. This is one of the great paradoxes that make music interesting. Pitch is a function of several physical attributes of a string: its length, thickness, material, tension, elasticity, and many more. We’re going to focus on length, tension, and thickness for our discussion here. All other things being equal:
Longer strings are lower in pitch; shorter strings are higher.
Looser strings are lower in pitch; tighter strings are higher.
Thicker strings are lower in pitch; thinner strings are higher.
So when you play an open G, you are causing a relatively thin, long, loose string to vibrate. When you play a G on the 15th fret of the E string, you are causing a relatively thick, short, tight string to vibrate. The pitch is the same, but pitch is not the whole story. Every factor we’re talking about here also affects timbre in nonlinear ways. And because each parameter changes depending on where you’re playing the note, we are led to an interesting conclusion: every instance of a given pitch on the bass is unique. This conclusion implies that for every pitch that can be played in more than one place on the neck, there is one “best” place to play it for every musical situation based on its unique properties.
The decision process you use to decide what is “best” depends entirely on your perception, so you need to develop that perception to make musically sound decisions. Of course, you also have to come up with a workable fingering, but the most efficient fingering is not necessarily the best for a given part. Here are two perception exercises you can use to prime the appropriate brain hardware for higher-level bass thought, and give yourself something to work on beyond pure efficiency. They are, on their face, boring, but at some point in your development you should spend at least an entire day doing them. You are evaluating physical qualities of the instrument that will inform every musical choice you make going forward; it is not wasted time.