How often should ducts be cleaned ?
At least every 10 years or less if you have pets.

Do you have to clean blower motor wheel?.
Yes the wheel is what moves the air through out your house and can be loaded in dirt which would cause all kinds of issues to your motor so yes blower wheel is a must clean.

Will duct cleaning resolve airflow issues?
In some cases yes and in some no. Airflow issues can be caused by broken duct work which we can replace or repair.
Also can be caused by poor design and installation which we can assess and repair.