What they said...but...
I HAD to grout my bathroom shower and floor in stages and couldn't do it at once. I was very careful to mix the grout the same consistency by measuring the water the first time and repeating. If you know where to look you can see slight variations in the grout, but with our floor tile, you don't notice, there's way more color variation in the tile than the grout (tumbled travertine).
If you decide to grout in stages...one thing you'll want to to is at every grout line, take a small brush and clean out the last little bit of grout. When you pack the grout in, obviously you work from the surface of the tile an push the grout into the line. Where the grout ends, the top will be full, the bottom will not. If you don't clean that small distance thats not full of grout, eventually it will fail. The best thing I found to clean the grout line out is the pointy end of a denture brush. It gets right into the crack, and with a slight pull you can take the grout back to where it's fully packed, which is where you'll stop. So from the side, the grout fill will look like this / if the line were being filled from the right to the left. Hope that makes sense...
