I suck at benching so take this with a grain of salt.
You may be over emphasizing pinching your shoulder blades, just like it's possible to over emphasize arching the spine. Do it to a point that you can still feel your traps, erectors, lats and chest working throughout the entire range of motion. You should still be bracing your core and keeping your legs tight as well. There's no point in getting into the most advantageous position possible if you can't even use your muscles in that position, unless of course you have such a big arch that it no longer matters. If you overdo the scapula retraction and can't use your chest, you may be moving your scapula into a position that allows you to better utilize your chest when starting the press up. At least that's what happened to me. Now I pull my scapulae in and down while judging how much based on overall tightness. Does it feel like your chest is tight or relaxed at the bottom?
For normal people, getting an extra quarter inch of arch or scapula retraction doesn't make much of a difference in leverages and it may be detrimental to muscle recruitment. Some people are just freaks and can get a huge arch while hitting heavy lifts but they aren't normally asking for help.