Well, I've always taught, and was when learning myself, that you should always be at circuit height BEFORE turning downwind.
The reason for this becomes very apparent particularly after having flown at some rather 'uncontrolled' airfields where joining procedure seems to be anything at any height.
Climbing into the downwind, particularly in a highwing Cessna creates a blindspot with regard to aircraft, especially low wing, joining the circuit higher.
ie: You climb and turn in a 152 and meet a Cherokee joining who can't see you either.
I'm sure though someone will have a reason why you should climb in the downwind, but personally I'm against it!
