There are two issues here: how much accuracy do you *need* and what can you do to maximize the accuracy you *get*.
As to need, I'll share what I've learned from HarrySin... With seasonal changes in humidity, wood expands and contracts in the hundredths (or more for wide pieces). Keep this in mind, even though you want to choose the tightness of the fit.
As to getting the most accuracy, some hysteresis (slop) exists in anything threaded. Its inherent with the clearances needed to permit the threads to turn. You can minimize this effect by putting gravity on your side. For example, with a router in a RT, always approach the cut height while raising the router. By this I mean that if you need to lower the bit by 1/16", lower it by 1/8" and then raise it back up by 1/16". This way the router doesn't lower itself in the slop during the cut. Gravity helps keep it from raising in any available slop.
Long dissertation for treating it like adjusting cut depth with a plane. If the cut's too deep, you back the blade off and them lower it to where you want it.
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