HD;
We also look at things a bit different in the US. First of all we don't differentiate with the RIS RAS and other such things <G>... You are either IFR or VFR getting some service. As to the mode C, we use it period. For IFR it must be verified for IFR use, however the FAA has done away with the not verified phraseology completely due to them feeling that mode C is mostly accurate. One of the things that we as controllers are required to do is if we see a mode C that is in error, we are requred to report it so that Flight Standards may have a chat with the aircraft owner about getting it fixed... They are fairly serious about that and it came about after the advent of TCAS. Something to do with some of the VFR's flying about with mode C showing FL315 and scaring the living blazes out of a flight crew...
Take care
Scott