Don't be afraid to charge extra for the first clean, you will be amazed, many people actually expect to pay extra when there windows are filthy, or you point out how dirty the frames are.
Before wfp many window cleaners did not touch the frames, only the glass, so nowadays they are getting twice the job done for the same price.
You will also find that charging more for the first clean, can sift out awkward customers, the ones that object to paying extra, are unreasonable and it will show up later, so save yourself some grief.