I'm Guy who mixes three bands at his place of worship on weekends + vocals.
The answer is, "It depends on what you want to do". But you will have to do a lot of measurements to find what you need.
One of our favorite pieces of gear has a switch on the input for +22,+4, 0, dBu and -10 dBV gains. Why? because there may be audio standards, but not every piece of gear follows them.
On a guitar pickup. first stage, gain of +10 to +15 and watch your input impedance, coupling, and roll-off.
Second stage, as much as you need to get to line level, and variable gain or fixed gain with a output potentiometer before the line driver.
Why, because I can hear the changes as the load on the pick up changes on low end and prosumer gear used by our musicians. Hell, even a direct box can color the sound.
I have one musician I will call "Dr. One Note Wonder", because of the horrible way his beat up old preamp / equalizer acts like a bandpass filter and peaks the middle of his main octave because of pickup loading. One day that box quit and we put him on a nice modern wireless sender with built in "classical music" grade preamp with very light loading. WOW, Night and Day.. Non technical persons walked up and asked him what he changed to sound so much better. However the old box is his "Security blanket" and he refuses to give it up.

We keep a tube follower around for those who want "that" hollow state sound.
Our digital mixer has a remote controlled variable gain preamp for each channel after the line receiver and then the slider on the IPOD we use for control. Both get used a lot on the guitars, and change a lot during rehearsals.
No two guitarists or bass players get the same settings on the console, and I'm trying to convince leadership that we need a "pickup" clinic some evening to clean up the mess between say seven different guitarists and their instruments.
Point of the matter, make your design very flexible. Cheaper to buy a few of them and see if they color your sound, versus building your own. Don't even get me started on the direct boxes that do not have a wideband transformer in them. I swear some of them use 60 Hz magnetics instead of audio transformers, and can see that on bench test when I sweep them.
Don't get me started on the scratchy, cheap, passive adjustment rigs in some of the guitars.
I'm not an Audiophool, my home speakers run on 14 Ga Zip Cord.

Steve