when i perform decline or incline bench i like to keep my arms at a 45degree angle from my body. if you are using correct form then i would say you are feeling it in your tri's because they are lacking compared to your chest. bench is a compound movement which envolves your chest, arms and shoulders. if one of these is lacking you will run into problems.
Take a look at the below links for your inner and outter chest that dont exist. the difference between flys and bench is flys are a isolation exercise and bench is a compound exercise. if you are looking for a more defined chest...ie line down the middle...then you need to focus on lowering your bf% and of course adding mass to your chest.
pectoralis minor
http://www.exrx.net/Muscles/PectoralisMinor.html
pectoralis major
http://www.exrx.net/Muscles/PectoralisSternal.html
my chest routine:
incline x4
decline x4
incline flys x2