Air is not designed to blow out the top. They are designed to pull air in the front and out the back. Stacking them is just fine.
Not experienced with rack servers, please advise.  Have a couple of PowerEdge servers R300 that use for testing.  They are just on the floor not in a rack.  Can  they be stacked on top of each other running.  I noticed that air is blown out of the top of them so I wouldn't think this is a good idea.  But if put in a rack aren't they pretty flush anyway?  Does is then just force the air to go out the back end, and there is nothing wrong with stacking them?  Appreciate the advice.  Thanks.
Stack PowerEdge on top of each other?