Yes, once your manager account's developer token is approved whether it is basic or standard access level, you will have the ability to read/write on all your customer accounts using the AdWords API. However, if the purpose is to provide read-only access to a third party, you will first need to
link them to your manager account and then follow these
instructions for granting the desired access for that linked account. You might also want to check a similar discussion in this
forum thread.
I hope this helps.