Have you ever noticed that in Inkscape when you have objects that are perfectly butted up against each other, that there is still a small gap rendered between them, exposing the background colour underneath?
This is a known issue with the way the SVG renderer in Inkscape does antialiasing, and there is even an Inkscape FAQ entry with a few ideas on how to workaround the issue.
However, I find that there is a simple workaround for this issue in a lot of cases. Simply group your objects together, and duplicate them twice:
One issue with this approach is that it doesn’t work very well if the objects you are using have any transparency, as you are duplicating objects below, which changes the way the top object looks. Also in drawings with many thousands of objects, duplicating them does tend to make inkscape a perform a lot slower. However in most cases, this technique works great as the finishing step in your workflow to remove the pesky gaps.