False according to MLB rules (if that’s what you follow). The rule states:
“8.01 Legal pitching delivery. There are two legal pitching positions, the Windup
Position and the Set Position, and either position may be used at any time.
Pitchers shall take signs from the catcher while in contact with the pitcher’s plate.
Rule 8.01 Comment: Pitchers may disengage the rubber after taking their signs but may not step
quickly onto the rubber and pitch. This may be judged a quick pitch by the umpire. When the pitcher
disengages the rubber, he must drop his hands to his sides.
Pitchers will not be allowed to disengage the rubber after taking each sign.
color=red The Windup Position. The pitcher shall stand facing the batter, his pivot foot in
contact with the pitcher’s plate and the other foot free.[/color] From this position any
natural movement associated with his delivery of the ball to the batter commits him
to the pitch without interruption or alteration. He shall not raise either foot from the
ground, except that in his actual delivery of the ball to the batter, he may take one
step backward, and one step forward with his free foot.
When a pitcher holds the ball with both hands in front of his body, with his pivot
foot in contact with the pitcher’s plate and his other foot free, he will be considered
in the Windup Position.
[color=red]Rule 8.01(a) Comment: In the Windup Position, a pitcher is permitted to have his “free” foot on
the rubber, in front of the rubber, behind the rubber or off the side of the rubber.[/color]”
for the complete rules.