Full compression of a bag is not a good practice, which is why many high end bags come with a loose storage sack, as well as a stuff sack, employed when the bag is actually out on a backpack. However, I must say that I stuffed down bags for quite a few years into a fairly compressed storage mode, and never noticed a big difference.

It is always good practice to unstuff the bag well before you will use it, give it a shake or two, and let it expand to its fullest before you jump in.