I don't always program Smith machine presses, but when I do I use a false grip. If you're accustomed to using it for the press, or want to take a walk on the wild side of highly non-functional training (I kid, I kid) the Smith machine can give you one hell of a contraction for the pecs and shoulders... but only when setup, bench angle, and form are right.
Many Smith machines have a predetermined bar path that's usually not straight up and down. Many have slight angles that bring the bar further down the chest as the bar nears the bottom position, and then brings the bar closer to the eyes at the top of the lift. This change in planes usually ends up with lifters flaring their elbows, protracting their shoulder blades, and losing all resemblance of upper back tension and stability.
We can avoid this by centrating the shoulders right off the bat, and allowing natural changes in carrying angle to happen without risk of forced and loaded internal rotation. Luckily, the false grip on the Smith machine limits unwanted positions at the end range.
Take your time on the setup and racking here since it involves some wrist flexion and extension to get into position to press. Make sure you have solid control of your hands, wrists, and forearms before pressing.