Its really two things.
Yes I prefer its forearms perpendicular to the ground, or angled very slightly out.
BUT this depends on were your elbows are which is the real issue here. Your elbows should be roughly on a 45 degree angle to you body, and not sticking out side ways at 90 degrees to your body.
At 90 degree it paces the stress on the shoulder, and involves a greater proportion of chest in the lift, and reduces any back involvment.
The closer the elbows get to your body, the more tricep involvement is required, such as a close grip (shoulder width) tricep press.
When arched up and a tad, and having your arms at 45 degrees, it also starts to use your delts, and your lats.
Your entire body should feel locked when you bench, ass clenched, and all the weight should be on your shoulder blades, and your ass just for stabilising, and feet driven into the floor, trying not to lift your ass.