To avoid delays or art modification fees, include bleed.

What is full bleed printing?

This means your want your artwork to extend to the edge of the trimmed page, so you must include a bleed in your artwork extending past the trim lines. If you don't include a bleed, two of the four edges will have an amateur-looking white margin after cutting. Also, if you put important words or art too close to the trim line, it may get cut off, so you should use a safety margin of 1/8" or 3mm.