Most wallpapers stretch a bit when they get wet with paste, and when drying, often there is a tiny bit (sometimes much more!) of shrinkage. This means you can end up with a teeny bit of your white wall showing inbetween the seams.
I hung this Thibaut pattern yesterday, http://www.thibautdesign.com/collection/old_collection.php?productID=1414&patternID=200 and I was pleased to see that the manufacturer had gone the extra distance to try to prevent this shrinking. A couple of things helped: printing the paper on a thicker stock (backing), and using a dark grey stock – the idea being, if it does shrink, you won’t see the white edges of the paper, but grey instead, which is less noticeable.