Now You Know!
From the August 2006 What's Cookin'

Reader Spreads or Printer Spreads?

There is a significant difference between the two. It is very important for the designer to specify the way a file is built to ensure that your file is imposed properly when we create a blueline. Unless noted otherwise, we will impose files based on Reader Spreads. Having folios on pages will also help ensure your project is imposed in the proper order.

Reader Spreads Reader spreads show consecutive pages on each two page spread, exactly how they look to someone who is reading the document. This is the preferred way to layout your pages.
Printer Spreads Printer spreads do not contain consecutive pages, but rather pages in the necessary order for the printer so that when the document is printed, folded and trimmed, all the pages will appear consecutively. You do not have to set your file up as printer spreads, our prepress workflow does that automatically for you.

They just called me a creep! (Also referred to as shingling)

These terms refer to the fact that the inner pages of saddle-stitched books are actually narrower than the outer pages. Each signature is nested inside the previous signature. In effect, this means that each signature wraps around a slightly narrower signature. Heavier paper stocks increase the difference between the width of inner and outer pages. The more pages of the book, the more pronounced the effect of creep is.

If you have folios, side bars, rules or any other repeating graphic element near the face margin, and you haven't adjusted for creep, the element in the center of the book will be closer to the margin than the element at the beginning or end of the book. If your margins are too tight, your copy can actually be trimmed off completely!

To avoid this subtract 1/32" from the face margin (move everything this far toward the gutter margin) within each successive, interior page signature. Keep each signature internally consistent, but make this change from signature to signature.

Perfect-bound books unlike those of a saddle-stitched book, are all the same width. Therefore, you needn't compensate for creep with these layouts.

Who's Bleeding?

Printing that extends to the edge of the page after trimming is called a bleed. To ensure ink coverage to the bleed edge, we require a 1/8" bleed on all documents. That means that as you build your document, you should extend color boxes and images to the outside of your document an 1/8 of an inch. This is a common mistake requiring additional work to correct.