I agree with both sides here, in different cases. In depends on what kind of images you're working with. If you're dealing with something like my avatar, which is a sharp bitmap with no smoothing at all, then obviously you need to draw it at the target resolution. Scaling will just blur it out.
But if you're trying to create something with smooth lines, it's often much easier to start with a very large image. That way you don't need to mess around with antialiasing at all; you'll get it all "for free" when you scale it down. I find creating antialiased bitmapped images to be a chore at best, because as soon as you want to change a single color....all hell breaks loose, because you can't just use the little paint bucket. It's MUCH easier to do your drawing with sharp lines, and smooth it later when you shrink it. For best results, you need to shrink it a LOT, though; 50% isn't enough to give you high-quality smoothing. And keep in mind from the start that you want it to be shrunken. For example, if you're target size is 50x50 and you start in 500x500, make your outlines 10 pixels thick. I use this method quite often when I'm designing OS X icons, and I also use it when I scan in pencil sketches and want to color them.