It appears that the "Remind me" field is not in the Settings options of the IPhone versions of Toodledo. For me, when I create a new task, the "Remind me" field is between Due Date and Due Time. I'd like to rearrange the field and give it an appropriate default, but, since it is not in the Settings, I cannot.
The iPhone app treats "remind me" sort of like a part of the Due Date field, so it will always appear right below Due Date. I have suggested to Toodledo that it would be more logical for it to follow Due Time for people who use that field. There is also no way to set a default for the reminder in the iPhone app, but it definitely seems like a good suggestion.