although you may not produce enough power during jumps/standing starts to pull out of your pedal, there may be enough leeway in your shoe upper that a strap may help with feeling more secure.
personally for me, im using shimano spd-sr's, the old 105 ones, there is a mod for them to attatch a strap to them which looks quite nice :
http://www.badbean.com/cyclingstuff/spd_straps.htm