On recumbents it is generally accepted that a higher cadence is better as it is very easy to overload your knee's, as you can use the seat back as a stop to press against.
Most on the trike forum I frequent run at ~100 rpm or more, once they get used to it.
As a bonus a higher cadence is suposed to burn more calories.
I have a slightly dodgy knee and it becomes very apparent if I start mashing to much, my cadence at cruise is between 105-120 rpm according to an exercise bike I was playing with last week, I was always a spinner even on df's probaly closer to 90rpm back then.
Oh, it's not just knee's that it can effect, hip's also, guess how I found out