[varispeed]

You need a third state to know that user has not made a choice. If you don't want to make a choice for the user, for instance, you don't want to default to F or T, then third state tells exactly that user has to make a choice.

[nescioquid]

Agreed. Sorry if I wasn't clear, but I consider that to fall into the "value is really nullable and you are using the presence of null to decide anything..." scenario, in which case, this is important to the domain, so explicitly model it so the next dev doesn't trip over the special meaning of null in this case.

Again, personal judgment and aesthetics, probably.

[withinboredom]

If it isn’t selected, the browser literally won’t send anything for that field (undefined in js parlance). Your code/language of choice has to make a decision on what to do with that state. If you specify true/false, the page will reload with that state instead of “undefined.”

For some languages it is exactly as you describe, for lower level languages (as in closer to the raw HTTP protocol) like PHP, you have to make an explicit choice before rendering the response.

[dathinab]

I would say using null as 3rd state is only ok iff it's part of the domain logic and the 3rd state is literally no choice was made, i.e. nothing, i.e. null.