A few specific tips:- on maps that have no AGT/Obelisk Engineers can rule. Maps like C&C_Islands and few others I've seen Engineer rushes take out a powerplant before the harvester returns to its roost! Sadly, part of the reason this works is because servers have no delay to wait for those with slower computing systems a time to load the map and get ready. So, in the first minute of these games those with the fast systems/connections can dominate. 
- however, on maps like the ones I mentioned its critical that technicians (especially GDI) plant proxC4 around the tunnels. Its unbelievable how quickly GDI go under when stealth troops come in with nuke beacons.