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