Arm position (in regards to parallel to the floor) means nothing, what matters is whether the bar touches your chest.
A good arch and a big chest will shorten the range of motion meaning that the arms dont go down so low.
benny, with DBs, be careful that you don't lower the arms too far.
In regards to arching... I have my feet up on the end of the bench to keep my back flat which I now see gives a bigger range of motion. I assume this is better for overall training as it will stimulate better growth. But in regards to 1RM would u suggest allowing my back to arch? I've felt nerves/muscles begin to cramp before which is why i've brought my feet up.