Meeb, the only thing I need is the cleared FL, plus any restrictions, e.g. FL150 lvl abeam SABER, plus all the other speed and heading stuff...
You are right when you state that Mode C is verified on 1st contact, which is why we always ask for passing altitude if you guys don't tell us. It needs to be within 300 feet of our secondary radar return.
On first contact pilots are supposed to report callsign, passing and cleared altitude and SID designantor. It is becoming more prevalent for pilots to miss out the passing altitude, unless you are Eastern European when it appears that simply callsign will suffice!!!