We are only required to issue traffic to the aircraft that we are talking to and have identified. If two VFR aircraft are about to converge and are not known to be on the frequency or have been radar identified there is no requirement that we reach out and broadcast a general traffic warning.
I do make one exception. We have a military low altitude route that runs through our airspace. If I see military aircraft usually A10 Warthogs (I haven't identified them and they are not on my approach frequency) in the VR route that are converging with another VFR aircraft (also not on frequency) I will broadcast on 243.0 (UHF Guard) that they have traffic. I do this because the pilots are already busy as all get out flying low level, and the route is near several of our satelite airports.
Mike
NATCA FWA
(A80 on Sept 5th)