I think there are rules about size, style of print, and even where gaps between the letters and numbers have to be.
Everything is supposed to be the same, basically.
Some people with no actual life try to re-position certain letters/numbers in an attempt to make it look like it spells out a particular word, like a personalised number plate - but to my mind if you have to mess about like that it will just look rubbish, as it's so obviously badly done.