Foot Placement
I prefer to place the feet flat on the floor with the legs spread wide. Many powerlifters prefer to tuck the feet under the bench to increase the arch in the back. Even though this does decrease the displacement of the bar, and increase the mechanical advantage, it is rather bad for the lower back.
For this reason, I suggest that you place the feet flat with the legs slightly straightened so you can drive your heals into the ground with out lifting your hips. When you perform the leg drive, you should feel the upper shoulders driving into the bench. This will not only help you stay tight but, will help you push away from your face, which is crucial for keeping the bar in its groove.
Holding Your Breath
The reason we hold our breath, or perform the valsalva maneuver (VM), is to support the spine and increase structural stability throughout the body. The danger lies in the fact that it can elevate the blood pressure to levels that some deem dangerous.
This is a very controversial subject, but is usually only dangerous for those with pre-existing conditions such as hypertension, or vessel weakness.
When performing the VM, it is best to take the breath in prior to unracking the weight. It is much more difficult to take in a sufficient breath when the shoulder structures are heavily loaded. Suck the breath into your stomach, and do not release it until you have locked the elbows back out at the top.