we leave a sippy of water in the crib...we used to do it just when she was sick, but it seem to help her sleep longer because she's been sleeping better (or settling faster when she wakes at night) and she drinks like half of it over the course of the night.
generally we don't do sippy cups or food unless DD is sitting, so I do wonder if it's totally safe for her to have in there as she might walk around the crib with it, but she is in a sleep sack so it's not like she's walking around THAT much (but she can walk in it).
I think the biggest safety risk is that they'd fall and hurt their mouth...I try not to worry about it because she seems to like having the cup of water in there.