Fontlab beginner question: should sidebearings be used for default spacing?


As a Fontlab beginner I'm still unclear what role sidebearings play.

Should they be used to denote the bounding box of a glyph, in which case letter-spacing is done entirely through kerning classes? Or should they denote a glyph's default letter-spacing, in which case kerning classes are an additional layer of spacing refinement?

Take the letter /I, for example: should you put the sidebearings adjacent to the edges of the glyph (so that the bottom-left corner has coordintes [0, 0]), or should you place them wherever you feel the default spacing should be (e.g. 50 pts away from each side of the glyph)?

thanks in advance!