According to one of my printers, the waste of paper is irrelevant. I would like, however, to see hints from others, to see what is the best decision.
Your printer shold tell you what size ranges work well on the sheet. My printer, for example uses a press sheet that can handle 4 9" x 12" pages comfortably on each sheet of the maximum size the press will take. going down to 8 x 10 results in no change in cost, but 7 x 9 might not fit so well on his press as it would on someone elses, so I might end up paying more because he has to run a larger sheet than really required (though he might be able to run a smaller sheet, too).
What he is telling you is that for any size sheet of paper, the actual dimensions of the pages don't matter, it's how many pages of that size will fit on the sheet and allow the necessary gripper and copntrol strip margins, etc. You should ask about the largest page size for the sheet, and work downward from that, then ask about the fit and whether reducing the size slightly from there would give a more efficient layout on press without sacrificing your design.
A few paper sizes come to mind that you could use but check with your printer on their thoughts as their equipment might be slightly different and they have to shave or add a mm here or there
Monarch is (w)184mm x (h)267 mm
Government Letter is (w)203mm x (h)267mm
Quatro is (w)229 mm x (h)279 mm
Keep in mind that Quatro is wider than 210 mm so it would probably come out of B sized paper rather than A sized paper.