I think there are benefits to both. Keeping your head on the bench and staring straight up is good for most. The bar naturally wants to go where you're looking, however, this requires that your bar path be perfect since you won't be able to visually track it. You also develop good neck strength. Personally I try and use this form as much as possible.
Conversely, looking at the bar on the decent I feel helps you wind up for the concentric which I've found helps me learn leg drive and really developing a pop off the chest. Like maraudermeat said, I also feel my front delts stay tighter this way, and I concentrate on pulling the bar apart since I don't have to worry about bar path since I'm looking at it. I use this more for heavy sets with a touch and go feel and feels more natural on boards as well. Sometimes I'll start a set staring at the ceiling and switch half way through based on fatigue.
Just my two cents. Find what works for you.