The insurance booth at Tangier Med said they only take euros! My mate had to get dirhams from the ATM at the terminal, then exchange them for Euros, loosing money and slowing the whole getting process of getting into Morocco.
If its closed they should still let you through, but the nearest places to buy insurance are (I think) the wafa assurance offices in either tetoaun or tangier port. If you get stopped for speeding they will expect to see insurance (spoken from a man whose been caught several times), so I wouldn't think about skipping it...
Also be aware that if you plan to head straight down south on the motorway that you will need dirhams for the toll booths (got caught out once). If you want to speed things up try buying some on ebay before you go.