This article on EDN breaks down why even X7R could be problematic in certain designs - basically they change value with voltage and produce harmonics. They were hitting it with max-rated voltage, but in a humble stompbox we're only talking about a couple volts - maybe 10% of max rating. The harmonics thing is interesting.
I've used them all over the place and never noticed any difference in sound, even in A/B comparisons. Might be enough of an excuse to start using the cheaper electrolytics more.
Anyway, here's the link (http://www.edn.com/design/analog/4426318/1/More-about-understanding-the-distortion-mechanism-of-high-K-MLCCs).
Excerpt:
" ... As a result, the value of the capacitor is changing instantaneously due to the applied signal, causing distortion in the current waveform. We demonstrated this by producing a SPICE model for a capacitor that replicates the voltage coefficient of a typical 50V X7R capacitor. The X7R capacitor models produced a large number of harmonics when used to simulate a Sallen-Key lowpass filter."
(http://m.eet.com/media/1202578/F8x600.jpg)
It really depends on what your requirements are. The highest harmonic is 60 dBV down, which isn't exactly close. I have seen the distortion in ceramic caps cause a problem, but that was on a device that was measuring at VERY low levels. Far below anything you'd see in a musical environment.
Voltage and Temperature can come into play. This is a pretty interesting article as well:
http://www.johansondielectrics.com/technical-notes/product-training/basics-of-ceramic-chip-capacitors.html