Why is 70% ethanol preferred for aseptic techniques?
70% ethanol is the minimum concentration when used in a lab setting for aseptic technique. You don't want to go lower, because it will not kill the microorganisms you are trying to kill.

However the solution requires water to be effective at doing its job. The 30% water is necessary to provide sufficient polarity to the solution. 

EDIT 


  A rough indication of a solvent's polarity is a quantity called the dielectric constant. The dielectric constant is a measure of the solvent's ability to insulate opposite charges (or separate ions) from each other...
  
  Water is the most effective solvent for promoting ionization, but most organic compounds do not dissolve appreciably in water. They usually dissolve, however, in alcohols, and quite often mixed solvents are used. Methanol - water and ethanol - water are common mixed solvents for nucleophilic substitution reactions.


Dielectric Constants


Water - 80
Methanol - 33
Ethanol - 24


-Organic Chemistry 11E T.W. Graham Solomons et.al.



Note

Dexter's answer provides experimental evidence for why 70% ethanol is used. My answer is based on my experience and understanding and the concentration that I have used in the lab. I would add that 70% was likely chosen as a balance between cost and effectiveness. If you are a little off on mixing the solution from 95% Ethanol stock, then you are still at a bactericidal level, where as if you err at 60% concentration, you could start to let a few bacteria survive.



70% ethanol is still flammable and should be handled with care. The issue with ethanol greater than 98% pure is that the only way to distill the last 2% of water out is to add benzene, and benzene is a known carcinogen. There will always be trace amounts of benzene in 100% reagent grade ethanol.