Body does by itself the stretch it needs in the morning and in the evening as well..so I think both times are essential, as the body is asking it. So my advice is, listen to your body.
I think a longer stretching session in the morning is good to get going, but I find a shorter session before bed good to!
I have heard that neither morning or evening is a good time period but I know that warm muscles are best, so if you wear warm clothing and gradually and lightly stretch to start out I think that is best. My neck is usually ready to stretch after 12 noon lol.