Start by clicking the 'random' link several times, to see how various mappings look.
Things to consider...
Blank rows
Consider the frequency of words that don't span all 5 rows.
Sparser mappings tend to have more potential ambiguity, due to it not being apparent where the top or bottom is.
For example, these 3 words could be confused:
acb, bdc, ced
Sparseness
Mappings that are overly sparse (fewer dots with more space in between them) may tend to make words appear less distinct from other words, and increase the incidence of words with blank rows.
Click the "sparsest" link to see the sparsest possible mapping.
Density
Mappings that are overly dense (many dots and less space) may tend to have a cluttered appearance. This can make them tougher to read, and require more finger activity when typing with 5 keys.
Click the "densest" link to see the densest possible mapping.