Q:

will the difference between a .001 cap and a .0015 cap matter? 

I am doing a build for a guitar stomp box from this website and was wondering if a .0015 capacitor could be used instead of a .001 capacitor.  The build i got from https://www.tonefiend.com//wp-content/uploads/DIY-Club-Project-2-v02.pdf and im using version 3 for the build

A:

Assuming you're talking about the indicated capacitor: 

It will make some difference to the frequency response (let lower frequencies through with the tone pot set to maximum resistance (CCW)).  
If you are blessed with a surfeit of 0.0015uF capacitors you can hack the correct value by placing two in parallel and then adding another in series with that parallel pair.  Total is 1/(1/0.003 + 1/0.0015) = 0.001uF. 

