Thanks for all the replies (although those on overhead joins are getting off topic)
Tony halsal said
I think that with a few more hours under your belt and a bit more confidence you might challenge an instruction like that next time.
I have about 200 hours over 10 years and I usually am asked to report downwind at my home airfield, an International airport in class D controlled airspace.
Nevertheless, the question still remains, if I had aligned with the downwind leg even 5 miles from the runway, I would still normally only call downwind left at the point where the crosswind leg meets the downwind leg. i.e. when the threshold of the reciprocal runway is at about 45 degrees when viewed from the cockpit window. This would still allow a potential collision with the crosswind climbing aircraft appearing to my left rather than in front of me. Other than actually seeing me approaching from the right, this aircraft climbing into the circuit would not know of my presence until I called "downwind left" at which point he could collide.
So we still come back to the issue that at this airfield and other similar ones, the ATC service is analogous to a FIS. It's still up to all of us to keep a VERY good look out.
Of course this entire thread could be repeated for base joins ie aircraft routing towards left base and reporting "left base" on arrival, risk collision with aircraft completing their downwind leg.
Finally, tmmorris's quote from the ANO was interesting. I didn't know about all turns being to the left.