The flight school I usually rent from is under the shelf of class C airspace. If you're going out to a practice area, the standard procedure is to contact approach control and get a squawk code, so they can better coordinate traffic. However, it's not required, since the practice areas are outside the class C.
At the moment, none of the school's planes have transponders*. One of the instructors said not to contact approach for now, because without a transponder, they wouldn't be able to do anything anyway and it'd just annoy them. On the other hand, during my last practice flight, I heard** them vector an aircraft around some unknown other aircraft. I suspect the unknown aircraft was me, which made me think that it would be better to talk to them after all. That way, I could (for instance) tell them my altitude, and they wouldn't have to vector an aircraft that was (say) 5,000 feet above me. But, back on the first hand, if I'm just a primary contact, they have no way of knowing which blip is talking to them, so maybe they'd have to vector the traffic anyway.
So, in that situation, is it better to contact ATC or not?
* Since this came so soon after the switch to ADS-B, and since it affects all aircraft at the flight school, I rather strongly suspect that they just got a bad batch of ADS-B transponders and had to send them back. That's just my theory, though, I don't know for sure one way or the other.
** I tuned in the approach frequency, even though I didn't contact them.