Use custom profile field in Rules
Under the Rules module add the event that fires After new user is created, then under conditions add a Data comparison, on the first step you should select your checkbox field, on clicking continue you will be presented with the options of this field, that is checked or unchecked.

Now add your action/s under the actions section.

If you want to also check when a user is modified, you should add a rule for the event Existing user is updated.