To be honest, I am not sure where exactly I got the idea of using the thumbless grip when doing my overhead press. It definitely was from one of the myriad of blogs I read. And there is quite a high probability it came from James Smith (the guy behind Diesel Crew).
Initially, I was a bit apprehensive about using the thumbless grip.
Thumbless = Unstable
At least, that's what I thought.
But I was pleasantly surprised that it made the lift that much steadier. The 'line of movement' of the overhead press felt smoother and more comfortable. Most probably because I could bend back my wrists a bit better. Bottom line... you should try using the thumbless grip for the overhead press. It's certainly one of the better tips I've read so far.