Notice we're also capturing the Marketo program that was responsible for the opt-in. As you can probably tell, our company has a very strict data privacy team - so in case we're ever audited (or someone complains), we can share both the date the user opted in as well as which marketing activity.
I would love more info on how you accomplish this. I am working on a new Opt-In procedure for our company now.
Like Dan, we also use a conditional visibility "progressive profiling" hack. I don't like how restrictive progressive profiling can be with ordering of fields, so we rely on if a hidden field has a value to show or not show an opt-in field.