When to mark an X

A cross-mark — most interfaces call it an X, some a dot — is a pencil mark that says "I've decided this cell is empty." It doesn't change the grid. Two solvers staring at the same finished puzzle, one with crosses everywhere they ruled out and one with the empty cells just left blank, would be looking at the same solution. The puzzle doesn't care.

You should still mark them. Here's why.

A nonogram is solved cell by cell, and almost every cell you fill in opens up a deduction in its row, in its column, or both. You'll come back to the same line three or four times over the course of the puzzle, and each visit you'll be re-evaluating the constraints in light of new information. Cross-marks are the part of that re-evaluation you can do once and then stop redoing. A cell you've crossed off doesn't need to be looked at again. A cell you left blank does, because blank is ambiguous — it could be unsolved or it could be a known-empty.

The cells most worth crossing are the ones you've ruled out on the first pass. The clearest case is a tight clue list — one whose runs and mandatory gaps fully account for the line length. A row of length 6 with clue 2 3 is exactly this shape: 2 + 1 + 3 = 6, so the placement is fully determined — cells 1–2 filled, cell 3 empty, cells 4–6 filled. Cross cell 3.

A second class of useful crosses comes from completed runs. If you've identified that the 3 in clue 3 1 is sitting at cells 4, 5, 6, the cells immediately before and after — 3 and 7 — are forced empty, because the run is bounded. Cross those off before you move on. They're the cheapest deductions you'll make all puzzle and they pay dividends when the column passes through one of them later.

The case for not marking crosses is mostly for the small puzzles. On a 5×5 easy you can keep the whole state in your head, and the cross-marks become noise. From 10×10 medium upward, mark them. From 20×20 hard, mark every one you can — your future self ten minutes from now will thank you.