I would say take the shot, but only if it meets the following criteria:
1-you are honest with yourself and know you are shooting within your limits, and you can make the shot.
2-the deer is moving extremely slow (I.e. If it slows down any more it is stopped).
3-it doesn't seem spooked or fidgety already.

My thought process on this is that if you stop him with a grunt, whistle, etc. This will peak his senses and is more likely to bolt, "jump the string" or whatever. If the deer is calmly grazing and seems relaxed, then make a judgment call, but definitely follow my first thought. (I'm not saying this is a 100% accurate thought, personal experiences will be on both sides of the fence on this issue. Just my opinion here.)

If anyone has more requirements before the shot, please expand on this.