So, in G/B, B is the descending bass line under the G chord. The G chord I normally play is 300023. So, is the last string/note played whatever is closest to string 6 or whatever is closest to string 1? Is this an understood rule? I mean how do you decide how to play the descending bass note? Why wouldn't I just play the B string, in other words?

When playing a G/B chord, the "B" refers to the bass note that should be played under the G chord. The specific order of notes in a chord is called the "voicing" of the chord. In this case, the G chord you mentioned (300023) is commonly known as an open G chord voicing.

As for the descending bass note, it is typically played with the note closest to string 6 (the thickest string), rather than the note closest to string 1 (the thinnest string). This is a common practice because it creates a stronger bass foundation for the chord and allows for a smoother melodic movement.

In the case of the G/B chord, playing the B string as the bass note (though technically correct) may not provide the desired sound and tension resolution. Using the thicker strings, like string 6, provides a deeper and more resonant tonal quality, which complements the G chord and adds harmonic depth.

Ultimately, there is no hard and fast rule, and you have some flexibility in choosing the voicing and bass note. It depends on the specific sound and effect you want to achieve. Experiment with different voicings and bass notes to find what works best for your style and musical context.