As long as it's covered and you check it every so often to add more water/stock, I don't see how it could be a problem... I've done it before!! I'm not 100% on that though so hopefully someone who knows what they're on about can help out
The longer the better I say, lets the flavours develop. I had a friend who would start her bolognaise sauce mid-morning and let it simmer away all day because she preferred the flavour.