in a compression pedal i'm going to build, i need a 50nf capacitor-will a 47nf one do or will it need to be 50nf?
47nF will be fine.
To put things into perspective, most standard poly film capacitors have a tolerance of plus or minus 10%. That means that any random 47nf capacitor grabbed from a bag could have a capacitance as high as 51.7nf, and it would still be within spec. Likewise, a 50nf capacitor could actually have a capacitance as low as 45nf. So, no it doesn't matter.