I’ve been looking into this a lot lately and it seems, as usual, there are many different opinions! Some advocate a ‘rack’ type position, where the bar is sat back in the hands towards the fingers and touches the collarbone/upper chest at the bottom of the movement. Others a more extreme ‘knuckles in line with forearm’ wrist position and some, as I do, feel it has to be somewhere in the middle.
When pressing (either overhead or bench) it is imperative that you can utilize as much of your strength as possible. If the wrists are rolled back, i.e bar back toward base of fingers, the wrists will flex and absorb some of the force. They will also create instability – akin to squatting in trainers (rubber soles). Aside from the pain you’ll feel when your using heavier weights, this instability should be enough to put you off, you don’t want to drop the bar on yourself! If you hold the bar with knuckles in line with forearm, then you are supporting the weight with your thumbs only – good luck with this with heavier weights!
However, when the bar is directly over the bones of the forearm (as in the right hand image), you can apply 100% of your effort directly to the bar. This can take some getting used to, especially if you’ve been letting the wrists roll back slightly (left image), however it is definitely worth doing. You will feel the difference immediately, especially on the bench, where you will get more pectoral activation and less deltoid. You may find you will need to lower your weights until you grow accustomed to the new grip, but your strength will come back and with improved technique comes greater muscle recruitment and thus more growth.
On a side note: don’t get confused by the term straight wrists, the weight of the bar should be on the heel of your palm so that you can squeeze it as hard as you can during the press. Give it a try next workout and let me know what you think?