Marshall Goff , Sep 02, 2003; 01:26 p.m.
There is a lot of information about this topic in the forum archives, so you can get something by searching about print resolution. You can also get some info at normankoren.com, luminous-landscape.com, and many other places.
That said, I'd strongly urge you to try it a few different ways and see where you start to see degradation in the image. I have not found that it makes a huge difference to print in an even divisor of the printer resolution because of good printer drivers (though I get that the math says it should), but I'm not familiar with how the Canons handle it (can't say good or bad). Here's a way to try this out, though: try printing the same image (preferably a good, sharp section) at various resolutions on the same sheet, side by side. Try, say, 150, 180, 240, 300, 360, and see the difference. Try it with the same image size, but different resolutions and try it with the same image piece, but let the printed size change with different resolutions.
Also, don't be terrified of upsampling. Good digital images can be upsampled to some extent before the quality loss becomes unbearable. You probably can't print your 3000 pixel image to 60" and view it up close without some problems, but you might be able to print it above 10" at 300dpi...