"I have a great way to make sure you do not mess up the exact pixel line up. It's all in how you start your piece. I always start by copying one of the adjacent squares to my clipboard. Then when I open a new file in photoshop it automatically knows to make it 150 x 150 pixels. Then I resize the canvas to 450 x 450 being sure to lock the tile in the right location. I can then copy and paste the remaining tiles, move them into their respective corners. Merge them into one and duplicate (that way I will always have a top layer that I never touch so I can see if what I'm doing is going to blend. When I get done making my own tile then all I have to do is resize the canvas to 150 x 150, again locking the proper sector. Boom. Save for Web and upload..."

Wow, thats a lot simpler than what I've been doing.

I feel stupid :(