I guess it depends on the pattern of error, entirely random error at such a low level is going to be inaudible but if you up the level a little and find out the error is modulated by something else on the board then it'll become audible much sooner than one might expect.
I know having worked through the circuit on my Hercules board that I'm better spending my time working the power supply for the transistors that drive the two sine waves than I am wasting money on better XO or power supplies for the XO.
Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m