One could ask technical questions such as, “Are the listeners ears within the vertical angle considered ‘flat’ for this speaker?” (Likely 10 degrees is certainly OK.) Or . . .

One could sit at the listening position, listening to familiar source material while someone else standing behind the TV changes the angle of the speaker back-and-forth toward being level and ask yourself, “Did that make enough of a difference to warrant adding a wedge?” Modified by: “Will the 20-inch TV be used short-term or long-term?”