I think it depends. How many other short stacks are there? If you are the shortest stack, the other short stacks will not play until you are anted out. When there are 20 and it pays 18 and you are so severely short stacked - you have to push, especially if you are one of the blinds. If you aren't in the blinds, the big blind should call you no matter what because he is getting good pot odds
When there is 19 left - you have to look at how many short stacks are left? Does anyone have less than you? How many postions are you from the big blind - how far are the other big blinds.
If you are having a significant number of bubbles - you are probably playing too tight in the late stages. Loosen up and try to steal the blinds of a tight timid player who has a medium stack.