They are PRE-LANDING checks, NOT "Downwind" Checks!! Eg, you might do a straight in approach.
That said on a square circuit, it is more common to make the initial flap selection on base (this assumes a powered approach, of course!).
As an aside in the PA28-161, I often see pilots hareing round the circuit at 110 kts, whereas I teach 90 kts since a) you have 20 kts less speed to lose on the base leg and b) if you have a PA38 ahead he will be doing 90 kts!
Hope this helps!