The 68k resistor on the input stage grid is there to filter out radio signals, this in combination with the grid input capacitance makes a low pass filter above the amp's audio range, but below the AM band. It helps if you happen to be close to a AM antenna (My wife's brother use to have a house near the WOWO antenna in Fort Wayne, IN and he would get radio on various bits of electronics in his house amps, phones, etc).
On later stages grid stoppers are used to prevent oscillations in the tube or to prevent blocking distortion. Aiken has a good page on all of this: http://www.aikenamps.com/InputRes.htm
Now, since you are talking about building your own amp you should also know that the 68k input resistor is also a source of 60Hz hum/ 120Hz buzz. It is a fairly high resistance in a section with low signal currents so it is susceptible to induced noise from your power sections (a little induced current over this resistance makes a large voltage change compared to your guitar signal). Sometimes you will find that you can reduce hum if you remove this resistor or reduce it to say 10k, if your not sitting under an AM tower your probably OK without it, if your Fender and want to sell as many amps as possible you probably want it. There is quite a bit of discussion on this on the AX84.com forum.
If your making a kit and the resistor is in the design the layout probably minimizes the hum effects if your doing it all from scratch its something you should watch.