Not sure about the assailants. They need to have the target in their forward hemisphere to fire at it and dis. missiles "nerf" the turning speed. So this may or may not cause trouble.

Still, the sentinels are flak frigs and they don't need to face any particular direction to bring their full firepower to bear.
The idea of turning discharging missiles ON when sentinels are stationary and on hold position has zero drawbacks and gives a serious advantage.
It's a no brainer.
Please, tell the Autocasting AI to do it.

You're right, the autocasting AI could use some love.

However, fixing most other abilities is a complex operation of prioritising some actions over other in certain situations against certain targets.
The case here is simple: Sentinel + not moving + on hold position => use Dis. Missiles.

When you issue a move order to a unit with Dis. Missiles ON with autocast, it turns them off, so no serious problem here even if there are no enemies.