I use 'allways' but I'm not 100%, it's the same with 'really'...
Fans 'seem' to work better pulling air through, they seem to make more noise which would indicate to me that more work is being done, more turbulance for example , which can only be a good thing...
For best results(according to popular concensus
) use a shroud and have the fan stand off by an inch at least to reduce the fan centres 'dead spot', for absolute bestestes* results use two fans shrouded as above in a push~pull configuration...
*
The Bogs Dollox 
...