If you're a 35-year old guy with an earring and gloves that match his weight belt, you put your feet up on the bench and hope the new 19-year old girl trainer will catch sight of your 'little lifter' flopping outta your short-shorts.
Otherwise, you plant your feet on the ground, don't wiggle them around during the set, and bench from a solid base.
Pretty solid advice from Dan John (and others): Jump up and down 3 times. The position your feet land after the third jump is pretty much where you want to squat/deadlift from. Most likely, this will have your toes pointed outwards just a bit.
I'd stand tall, definitely not slouched forward, but not exaggeratedly arching back either.
Worry about these nuisances like technique...yeah, that's definitely worth checking up on, no problem.