Advantages and disadvantages of each method in my opinion:



Method 1

+ Completely specifies each row/column

- Difficult to remember the 6-character legend



Method 2

+ Simplified version of Method 1

- L appears twice as often as I, making it somewhat guessable



Method 3

+ More consistent with other shading answer keys

- 1 and 2 can't quite differentiate I's and L's



Method 4

+ Centers are not easy to guess

- Probably difficult to notice



I myself seem to prefer Method 3 slightly, with Method 2 coming second.