If you have a relatively long body compared to your legs, you may need to sit further behind the BB to get a good balanced position compared to somebody more "normally" proportioned. If you use TT and stem length then you'll end-up with lots of weight on your arms, so some of us find layback posts pretty useful.
My TCR has quite a layback post as it has quite an upright Seat-tube. My Brooks saddle sits on the layback post almost as far back as poss. Stem length is 100mm, I'm very comfy!