I think it is always very prudent to have atleast one joke up ya sleeve for any/all situations, the one I always remember is :
"There were two parrots sitting on a perch"
One says to the other.....Can you smell fish?"
This is usually greeted with a look of.... I don't get it, then after about 10 seconds the penny drops that a perch is also a type of fish.
thing is with this joke, its quick so you have little change of F*cking it up, and the person who asks you to tell the joke has to do a little thinking
Its a good one for an interview situation.