The bruising is not always the same. It's due to the amount of psilocin in the mushrooms, and as you would expect, this is not always the same. Some mushrooms (particularly younger ones) will bruise very heavily after even a couple of minutes. Decent sized mature ones will often have signs of bruising already before you pick them, whereas old, drier mushrooms will not bruise particularly obviously. Either way, your best bet is to nik/scratch the stem, as sub stems are white, so the blue bruising shows up very obviously.
If it's not noticeably blue, don't eat it, be safe.
I'll take photos of bruising next time I pick some subs up.
As for chopping/not chopping, i chop, because it's the best way to determine a safe and reliable dose. If you have all your mushrooms (which vary widely in their potency) all chopped up and mixed together, then each dose of the same size will be pretty close in potency to the last. Personally i'd prefer safe doses to a little bit of oxidation any day.