The difference between chord-line angle of attack and zero-lift-line angle of attack depends on the airfoil’s camber; and the camber of a horizontal tail surface changes every time you move the elevator. The angle of attack of the empennage is also affected by the downwash of the wing, which varies with speed. So a monoplane really doesn’t have a fixed decalage, except in the trivial sense that the stabilizer is attached to the fuselage at one angle and the wing at another.
Stabilizer incidence, like fuselage incidence, has a small but detectable effect on drag. If you have a fixed stabilizer set at the wrong angle for cruising, you have to correct by trimming in some elevator angle. As you might guess, a surface like a horizontal tail, composed of two elements hinged together, has the least drag when the movable surface, the elevator, is “in trail,” that is, aligned with the fixed surface.
Many airplanes — Cessna 180s, Mooneys, most jets, all airplanes with stabilators — have adjustable stabilizers, and so never pay a drag penalty for a kink in the horizontal-tail airfoil. For them, the decalage has no fixed value.
Fixed stabilizers, however, sometimes present designers with a problem related to control. The correct stabilizer angle for cruise may not provide enough elevator authority to rotate for landing with flaps down and a forward CG. So it’s necessary to set the stabilizer more nose-down than the ideal, and then to trim the elevator downward for cruise. This is the arrangement on Cessna singles, from the Skylane up. You can see it if you look at the horn balances at the outboard ends of the elevators. They are angled downward in order to line up properly with the stabilizer when the elevator is trimmed downward for cruise.
The fact that incidence and decalage don’t have fixed, ideal values has an important corollary. It implies that you can get things wrong and still be safe. The deck angle may be funny and the elevator may not line up nicely with the stabilizer, but the airplane will still fly more or less correctly.
This is true of conventionally configured airplanes, but not of canards. An argument often made for canards is that the stabilizing surface contributes to lift, whereas in conventional airplanes it usually (though not always) produces a downward force that makes the wing work a little harder. There is a collateral benefit: Since stability requires that the canard produce more lift per unit of area than the wing, the canard will normally stall first, and thereby protect the wing from stalling at all. But if you get the decalage wrong — and a number of designers have — the wing may, under some circumstances, stall before or at the same time as the canard does. Usually, such a stall is unrecoverable. That’s the reason that amateur designers, however great the attraction of the canard, are well advised to stick with the conventional aft-stabilizer arrangement.