One thing people often say to me when I'm explaining my system for turning three-digit numbers into images is "Surely it makes more sense to have 999 be a policeman [or 112 or 911 or whatever the number for the emergency services is in the asker's country] and so on?" I know Dominic does recommend diverging from his system if the number itself suggests an image to you, but I disapprove.
The 'Ben system' and any other system for turning numbers into words and thus images is best compared to learning another language. If you've got it working properly in your head, you don't see the numbers as such, you read '999' as 'b-oh-b' and think of Bobo (Mr Burns's teddy bear) or whatever your favourite word starting with that sound is (I recommend the cartoon series Bobobobs, or the German children's book series 'Bobo Siebenschläfer', but I'm just weird like that). Reading it as a number in some cases and a word in others is just confusing.
Look at it this way - if you speak French and read the word 'pain' in a French text, you don't automatically think 'ouch', because you're mentally pronouncing it differently and taking it in context. That's how it works with reading a story in a 2000-digit number too.