Ideally the impedance buffer should be placed as close to the pickup as possible. Before any switches, and also before any significant length of cable.
A piezo is a voltage source with internal resistance (R) and capacitance (C) characteristics. The buffer terminates the RC circuit of the piezo with a high impedance and isolates it from the RC characteristics of the anything that follow the buffer. This effects the piezo's frequency response, damping, and low frequency cutoff.
Switches and wire all have their own internal RC characteristics. These may impact an unbuffered piezo, but they will typically not have an impact on a buffer unless there is a problem somewhere after the buffer.
From practical perspective, I can't say if the impact/degradation will be significant enough for you to care. AFAIK you won't hurt anything, so try it both ways and make your own decision.
For the record I have used a passive ABY to switch between upright and bass guitar. I was still able to get a decent sound out of the upright.
Around the same time frame, I did some experiments with lengths of 1/4" instrument cable and found that I could hear a loss of highs when the cable length extended beyond a few feet. I wanted to be able to move a around a bit, so I used a really long cable. The impact on my tone was noticeable, but I simply turned up the highs on the amp a slight amount and it was fine.
