Sometimes I find I have to want to work on a piece instead of always sticking to a set plan.
My thought (and personal experience) is to do the newest piece first while my brain is fresh then work on one giving me trouble at the time (or the parts giving the trouble).
I think it was in the book The Musicians Way that I read advice to vary when you practice pieces, and that is what I have been doing. I basically let my mood of the day dictate the order. I found that if I was following a certain order, say starting with the most technically demanding pieces and ending with maintenance work, there were days I wasn't mentally ready to start with the demanding pieces. Some days you want desert first, and I think your frame of mind being in your work is more important then say being fresh for the demanding work.
When you have a number of pieces in various stages of learning, what order do you practice them? I'm kind of all over the place. Do you feel it matters?