Some home bike-mechanics use grease like whipped cream; the more the better. It really serves no lubricating purpose on the steerer but, if used sparingly on the steerer, it will act as a sealer against moisture. Also, it is a bit of a solvent so I wipe the steerer with it before installation. Alloy=aluminum so rust isn't a concern but a small level of corrosion could occur with a different alloy composition of stem and spacers (Galvanic series and all that). The thin coat (above) solves that problem as does the carbon spacers (use them--cool little item). Fine points: grease attracts dirt like a magnet, holds moisture forever if it gets trapped in the gobs of grease (bike on a car rack in an unexpected shower at 60 mph) and it gets all over your hands and everything else when you're working on your bike. Use it where you need it and then only in the amount you need.