It depends on the puzzle. I X all the boxes for the 5 people clue, always. That always helps me later on. I always mark all the negatives in the first row. I will do the ones below the first row if it helps me. I'd say that it's about a 50-50 split for that. If I have a lot of information in the first row very quickly, I won't do the 2nd and 3rd row of the grid, just work from the top row. If I don't have a lot of information in the first row, i.e. it's all spread around, then I will use the entire grid. So it depends on the puzzle.
Hope that helps.