If you measure tiller to the bowstring you might get a false reading depending if your cam and idler/cam and 1/2/or two cams are not the same diameter.Better to have a string from axle to axle imo.

In a perfect world where all limbs are perfectly matched and all parts are machined exactly the same, then that would be the case. Unfortunately, we do not live in a perfect world. Don't count on this old wives' tale to work for you.

Also, it is not always a given that a bow will shoot it's best, whether talking speed or accuracy, with the limb bolts bottomed out. This is another myth that's been repeated so often it's become intrenched as 'truth.' I've seen some bows that picked up anywhere from 5-10 fps just by backing the limb bolts off a half turn.

First item on the agenda when you tune a bow is to set it at your target draw weight. Then adjust tiller where it measures the same, top and bottom. That is your starting point. (If your cams are the same diameter top and bottom, then you can measure to the string. If not, especially true with single cam bows, then follow jsasker's advice on measuring.)

Then I set nock height and centershot for a rough tune.

After doing that, I readjust the tiller so the bow fits my grip. Each of us grips a bow slightly differently. Some have more grip pressure in the grip's throat, some more pressure at the bottom of the grip, some keep the pressure fairly evenly distributed along the grip. This causes the bow's limbs to react different for one shooter than they would for another. The idea is to match the limbs' action to your individual grip pressue.

So, I hold the bow straight out, eyeball a sight pin at a spot on the wall and draw, slowly. I pay attention to how the bow reacts. If the bow pulls the sight pin UP, off my spot, then I take a little off the top limb bolt and/or add to the bottom limb bolt. If it pulls down, I take a little off the bottom limb bolt and or add to the top. I adjust until the pin stays fixed on my spot all the way through the draw. (If you're overbowed, you won't be able to do this step.)

Then, and only then, do I proceed into my fine tuning program.

This method of setting tiller makes for a more pleasant draw and rock solid aiming. I also feel it greatly simplifies tuning with broadheads and improves broadhead accuracy.