Asking the employer if it would be ok to call them in case you do not hear from them is a great way to ensure that you will be able to call them. Most people will avoid being rude and hereby you validity that you are able to call them later on. Staying in contact throughout the process is a great way of making sure that you do not get forgotten or disregarded the opportunity because they think you are not interested in the position.
Opting-In to Follow up: Your intent with this question is to get the employer to opt in to you contacting them again. Some candidates use this as a method to evaluate whether or not the interviewer is truly interested in them.
All Questions to Move the Process Forward during the job interview
All HR & Recruiter Questions to ask at an Interview