I varied mine over the years for at least awhile at each of the settings that has already been stated.
The biggest indicator of what worked best at any given time was the tires I happened to be using.
At one time or another then, it worked "best" with each one of those measurements. Most often the 3/16", but was acceptable at all.
My recommendation then would be to pick a number, then drive it. Come back a week later and reset it to something else, then drive it again.
Easy to do when it's your daily-driver, of course, but still doable when only driven on weekends. Just takes longer.
I'm guessing that the amount of caster and camber your particular truck has, as well as the choice of tire, size, tread design, and chosen air pressure, all have an effect (however slight) on what's going to work best in the toe-in department.
That's not a copout, it's just a fact of life in some cases. If you have the time then, start at 1/8" maybe, and then move up and down until you find the sweet spot.
Paul