nh
Sarking is definitely needed for colorbond, its actually illegal for a plumber to install colorbond roofing in Vic without the metal foil in place . If you have colorbond roof and the foil is missing then its been installed incorrectly.
Sarking is not needed for a tiled roof because there is enough gaps between the tiles for the condensation to dissipate.
If you live anywhere where the summer is hot then I would get reflective foil put under the roof when it gets installed. This can significantly reduce the cooling load on the house and reduce the cost of running air con especially ones with ducts in the roof cavity.
It is much easier to put the foil in at time of installation then afterwards. It's cheap as well $2-5/sqm.
Moisture control and condensation is much more involved issue then just putting sarking down.