I print 24x34" posters and what I have found best is, that if you are using a graphic as a background, your first step should be to resize to the correct size and resolution. I find Perfect Resize (used to be Genuine Fractals) invaluable for this. I match the resolution to the printer or the driver. In my case, 300 dpi works well. As you drop in other elements, you'll want to resize them to the same resolution, and adjust the size as needed separately beforehand. If you start with the background at your target resolution, all of your type will automatically fall to that resolution, and you won't get jagged edges. Finally, when you are ready to make a proof, just reduce the size to something small to save paper using Photoshop. Print you proof, but make no changes until you hit the "Step Backward" selection in Photoshop in order to bring your poster back to its proper size.