As I and others said you can eventually be refused if going with a tourist visa or exempt if going to often, the IO will decide on the day like it or not it happens.
I have forgotten to do mine and done it online a week later. Never been an issue for TM30 it's not that important if your have no need to go to a immigration office for anything.
Do you need a visa ? How long are you coming for ? Currently you get 60 days plus extension of 30 days having a USA passport. Depending on your arrival date you may not need a visa.