The old school amps used 68k, largely so they could make the 2/1 voltage divider between the two input jacks - those high and low sensitivity jacks. SOme reissues, like the Twin AMp used a single 33k to emulate the typical 68kx2 in parallel.
A quick tour through my Fender disc shows most of them using a 10k grid stopper, rather than 68k. When there is only one jack, there is no longer a need for the 68k resistors.
And I don;t know what you mean does it boost signal too much? Resistors are passive and do no boosting. But in the larger sense, are you suggesting that Fender just stuck a random value resistor in ther? Or that they designed this without knowing what they were doing?