Bob, They will take anything at the borders, including sterling- I carried a couple of hundred euros on me but just withdrew dirham at ATM machines (plenty around), so cannot comment on exchange rate at Tangers. Make sure you have good supply of small notes and coins - always useful for tipping anyone who gives you service (eg petrol attendants etc. etc.!) Worked generally on 16.5 dirhams to the pound, or 10 to the euro - as an example our hotles were between 160 and 460 dirhams a night for a double room; meals btween 60 and 160 dirhams and petrol was 10 dirhams per litre. Took green card with me but was never asked about insurance and would certainly not be forced into buying it at the border.
When we arrived at Tangers there were several characters wandering around the port with blue waistcoats which said 'Escivains Public' on the back - these will try to 'help' you by getting you through the police checks and customs quicker, with ensuing 'gifts' for them and the customs guys - if you want to get through quicker it is worth carrying a few 5 euro notes. They seemed to target bikes because it was easier for them to get out and around the queues, but maybe we just looked easy pickings! There are lots of people around in different uniforms so it can be difficult to tell who is genuine and who is trying to con you, but hey, welcome to Morocco! At the end of the day you will only be a few quid out of pocket but richer in experiences! (Well, that is how I looked at it - 10 euros in our passports on the way out did get us on the next ferry a lot quicker!)
Have a good trip!