I used to have an arrow knocked, thinking it would save me from being busted when the time to prepare came.
Authors of books/magazines convinced me that it was too unsafe and that only fools do it. So...out of shame, I quit doing it. I've found that I almost always have time to get an arrow out and nock it without spooking a deer.
Is it really that unsafe? Probably not - you are going slow, but there are some trip hazards and such. (Remember that I hunt fairly flat ground and not treacherous slopes.) The real chances of cutting or impaling yourself are slim - but since it is fairly easy to nock without detection, why take the chance?
Also, the chances of dulling your broadhead on honeysuckle or brush/leaves/dirt, as well as the noise made when it catches on limbs or when it falls off the shelf are higher. You also have to make unnecessary motion to navigate the long arrow through brush, which increases the chances of being seen.
That's my opinion for what it's worth