I've seen some gnarly developing trays from this approach as they stain and wear easier. But I have never seen them negatively affect the prints. As long as you cover them somehow, I see no problem as long as you don't plan on reusing it for a prolonged length of time. I have one of those big tanks with the spout and a floating lid, so for the 5 seconds it takes to dump it in (even my 20x24 trays), I see no point in leaving it out and diminishing the life of the developer from exposure to oxygen. I can use the same 2 gallons of developer for maybe a month, but if I left it overnight a few times it would be pretty close to done. Economically, not the best idea.