There are two considerations:
1. How much DC current do we need to bias the transistor?
2. What load do we want to present to the previous stage (or device)?
The base current flows thru the bias resistor and we don't want too much voltage drop.
Often, that bias resistor is part of a high-pass filter. We choose the R & C to pass the right amount of bass.
An example is in order. Let's look at the LGSM TS clone input buffer. Vref is 4.5V and R3 is the bias resistor. Q1's collector current will be around 400μA. According to the datasheet, median HFE at Ic = 400μA is around 125. That give us a base current of (let's not see all the same hands now...)
Ib = 3.2μA.
That means we drop 1.6V in R3. That voltage drop eats some of our headroom, but we have plenty of headroom on the input side because a guitar signal might peak out a 1Vp-p if you have hot pickups and hit the strings real hard. 510K is a high enough resistance that we won't load the pickup appreciably.
Now let's look at the output buffer. I included C7, C8, R11 & the VOLUME pot because I want to suggest an improvement. R12, R13 & Q2 are the same as the input buffer, so we drop 1.6V in R12. We will have the same or more headroom if we set the VOLUME for unity. But if we dime the VOLUME, we'll have less headroom. Probably not a big deal if we keep TONE below noon because the max output from the 2nd stage will be on the order of 2Vp-p. But... There is no problem with loading the volume pot a little bit, so we could reduce R12 to 100K which in turn reduces the voltage drop to 0.32V and increases the headroom. We could also use a higher HFE transistor for Q2, which would also reduce the voltage drop in R12. We could go a step further. The output of IC1.2 is biased to Vref. The VOLUME control is biased to Vref and so is Q2's base. So why do we need C7, C8 & R12? We don't. We can remove all three parts and jumper C7 & C8. Now the maximum resistance between Q2's base & Vref is 100K. As we turn VOLUME down, the resistance gets lower. Why are C7, C8 and R12 even in there? Because they are leftovers from the Tube Screamer's soft switching. There
might be some slight scratching when VOLUME is turned since Q2's bias will change a little bit when VOLUME is turned. Probably not audible and even if it is, so what? How often do we turn the VOLUME knob?