Wing Chun’s “Mother Stance”, Jing Mah, is not a fighting stance.
The function of Jing Mah is to build basic structure. It teaches balance, structural concepts, and is used to learn various hand positions. Jing Mah teaches basics that are later applied to movement and other stances. It, itself, is not the end goal – it is the first rung of the ladder.
Usually, Jing Mah is not used in combat, though a fight may start from this basic position. The typical street fight begins with a face-off. This would be the time to assume a position something like Jing Ma, to ready yourself while avoiding movement into a well-defined fighting stance, as doing so would only guarantee a fight.
That said, blading your stance – moving a little outside of the opponent’s center and turning your body at a slight angle to your opponent – is a better initial position. In a bladed position, you might use something akin to Jing Ma, or more like Ju Sun Ma, (Sideways stance), Ding Jee Mah (T or J shaped horse stance), or Pien San Mah (side body horse stance) – all of these are minor variations depending on family.
During a face-off assuming an “On Guard” position, with hands in fighting position is likely to provoke the fight. However, being in a usable stance with arms ready may help to diffuse the situation, and lends an element of surprise.
While bladed stances are the most effective in combat, if you cannot execute Jing Mah well, you will have difficulty using and moving your Bai Jong (fighting stances).