Er, I got a switch in one guitar which I can use to swap over to different caps.
I cannot hear a blind bit of difference between finest polyester and a tested same value ceramic, nor can I see anything when I analyse the spectra in CoolEdit.
I bought a lucky bag of ceramics from Tandy (Radio Shack) years ago and I haven't run out yet, but I will admit to using mylar or better in my guitars because they are more consistent. I always use an audio or log pot for tone because I find them to have a more usable gradual range, whereas the linear type give me nothing, nothing, nothing, nothing, nothing, mud. But hey, ymmv.
There is some difference in frequency response in cap types but only when you get into r.f. at vhf and uhf, I'm reliably informed that ceramics become useless then, but to 20kHz audio... ?
There's two kinds of people, those that hear the music and those that don't.