I don't feel like it is fair, so I voted no. This is why I try not to accept money before I'm ready to begin someone's commission. I hate to make people that have paid wait long periods of time. Not being paid until ready helps to keep things flowing evenly when I work too :)
If I were to begin doing weekly streams, I'd make sure to get anyone that has paid done first.
I also feel like artists can get behind much more easily if they get paid by everyone first, since once you have the money you need, it can make people a little more lazy about getting things done. Then if you fall behind on commissions, you already need money again and then accept more commissions for money, and this is how backlogs of commissions happen!~