Personally I like all the veins going in the same direction. It's like the tiles were cut from one slab, or "sequential" slabs. Some people pay extra to have their marble floors or walls matched booked.

Im the opposite. I would try to randomize them.

I am a whacko and would not be able to put them all the same direction because the veins wouldn't line up perfectly and it would make me crazier than I already am. I would put them down in different directions.

I'm not a fan of random but since there are only 4-5 patterns you have to be careful that it doesn't look fake if you have them all going in the same direction. If you're not careful you'll make too perfect a pattern which would draw attention to the fact that it's not real marble.

Someone else posted pics of theirs (I saved them to my phone I liked them so much):

That's absolutely gorgeous IMO! Random would make me dizzy.

i put them randomly and it looks better. My builder marked one tile with a marker and it seeped right through the tile. Everything else washes off (pencils/crayons). Has anyone had that happen? would a sealer help?

It really depends on the tile and the pattern repeat. My tile guy laid out my 24 x 24 shower wall tiles on the floor first so we could come up with a pleasing layout. Mine mostly go in one direction. It is similar to a puzzle and laying them out takes time, but well worth it for a result you will be happy with for years to come.