hi cynthia,
i've done this procedure many times with lots of different cell lines.  we
used them for ihc controls for abys that researchers would bring and ask us
to stain various mouse tissues (they usually couldn't tell us where to
expect to find staining).
we spun the cells (numbering at least 1 x 10E6 for a good pellet) for 10
min. at 5000 rpm (if i remember correctly).  then the pellet was fixed
overnight in 10%NBF.  the pellet was then very gently "scooped" from the
tube they were spun in and wrapped in lens paper and processed as usual.
please be aware that not all cell lines will pellet well.  some cells seem
to compact and stick together better than others.  this can make it
difficult to remove the pellet from the tube intact.
good luck,
carrie kyle-byrne
eire@teleport.com