Why am I getting messy gaps between my 1x1 tiled cubes?

I'm using scenery which is a set of tiles I made in 3Ds Max. I'm pretty sure they're all perfectly square and all fit the same grid, but you can see gaps appear in the scenery when moving around. I've placed these using vertex snap, which seems to snap them together really tight, but you can still see gaps.

Is there a way to do this better? Placing them with tiny overlaps is driving me crazy tbh!

Here's an image of what it looks like http://i.imgur.com/qP8Y1E8.jpg (although difficult to catch the majority of the gaps which are only obvious when moving).