The most accurate information for a product is going to be what the manufacturer presents on the box, bag, or can. I agree it's a pain to always have to enter custom foods. If you want to be accurate, it's a fact of life, though.
I've noticed over time that some foods change and even my custom food is not exactly right anymore. McDonald's filet of fish one day turned out to be smaller than my original entry's information - good news, but it meant I had to change my custom food.
What that means is I try to spot-check custom foods. It only helps to be more aware.