Pete S. , Mar 04, 2009; 06:17 a.m.
You can spot meter on anything but you need to know what the tonality should be. For instance if you spot meter on the dress and it's white you want it to show two or three stops over middle gray (0). If you spot meter on a black tux you want it to show two or three stops under middle grey. A caucasian face should be about 1 stop over.
In general spot metering is usually used with manual exposure mode. Unless you feel very comfortable shooting manual mode and using spot metering you might be better off letting the camera do some of the work for you. For instance in matrix or center weighted and perhaps aperture priority for exposure mode.