It is common practice to use a round dot shape for flexo. FIRST specifies the use of a round dot. A true round dot does not change shape and continues to grow throughout the tonal range. A round dot does not touch its neighboring dots until the 75% area, unlike a Euclidean (round-square-round) dot, which touches its neighboring dots at the 50% point.
Euclidean dots touch at 50%
Flexo round dots touch at 75%
In flexo, round dots are used to move the visible tone jump, created at the point where neighboring dots first touch, into the shadow region where the tone jump is less noticeable.