What effect flap deployment has on pitch trim depends on several things. Deflecting flaps results in a nose down pitching moment on the wing, however at the same time the air leaving the wing is deflected more sharply downward. If the horizontal stabilizer is in this area of increased downwash, the increase in down force on the tail will tend to pitch the nose up. It will also be affected by the location of the cg. A more rearward cg will usually cause a nose up moment while one more forward will cause the nose to pitch down. The final net pitching moment is hard to predict and depends on which of the several effects wins the tug-of–war. It’s not so much a design thing as a try-it-out-and-see thing.