Suppose a monster has Reach and is two spaces away from the nearest hero. It's action window is (1) Engage the closest hero (2) Attack the closest hero.

RAW seems to indicate that action (1) should occur since "when engaging, a figure is trying to get adjacent to a target" [RTL rules, page 8], even though with Reach it could attack from where it is.

The reason why it could matter is if action (1) is first skipped and the monster defeats the hero with (2), then it could go back to (1) and move towards a new hero.

Go for adjacent, if possible. The app have special activations for the monsters (especially masters) with the reach skill in mind (or others like throw).