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