I am preparing data for the ISC census and having to go back and enter EHC Plan = No for the majority of pupils added to the register in the last twelve months. The SEN teachers do always set the field to Yes where a plan exists. I can't foresee a situation in which it was not known that a student has an EHCP so it would be nice to make sure the field was populated at the time of adding a pupil to the register, with Unknown as an option if really necessary.
This feature is already there in the Enterprise version, where you can set the field to be required. There is also the ability to batch/group update the data in the Standard and Enterprise version,