It's what smooths terrain next to cube blocks as well, so that you don't have that gap. It's also what causes those "dorito tears" you get sometimes when digging, because not only does the block density make a block look a certain way, the block NEXT to its density does as well.
You can cause "texture bleeding", too, which is a cool effect... like say for railroad tracks, I took the wooden poles and "sunk" them into the gravel ground so that they weren't so... blocky.
Definitely trial and error. =)