For the terrain I'm working on, I'm using only default textures. But, I'm using a lot of them...farmland, beach, swampland, dirt/rocks for hillsides, snow for high peaks and for coral reefs, etc.
I know the more custom textures you use, the bigger the terrain file - but I'm not sure about that if you're only using the default textures.
You can also select two textures and they will blend into each other, which makes transitions from one area to another look really nice. Sandy grassland, rocky snow, swampy beach, etc.