Don’t let your dog approach. Even if both are friendly, it is an unequal meeting and not fair to the on-lead dog. There is usually a reason the dog is on a lead: respect this, give space and move on.
2. Both dogs on lead
Ask permission first and wait for the answer. Even if the owner says it is OK, be careful: being on leads adds extra tension to any meeting. If the answer is no, respect this, give space and move on quickly and quietly.