If the B is a single stemmed note, i.e. not beamed together as in eighth notes, the stem goes down. If it is beamed, like eighths, sixteenths, etc it depends on what the other notes on the beam are. If the majority of the other notes are above B, the stems go down, and vice-versa. In a choral score or other scores where there are tow parts on a single staff, upper part (i.e. soprano) goes up, lower part (i.e. altos) go down. Also, in non-beamed notes, a series of quarter notes for example, the beam may go down if it is part of a pattern for clarity sake like: G G A B G in a repeated pattern. Clarity for the reader is your guide, not convention alone. Rachel Lee Los Angeles