Relationship between chain tension and chain wear On a 10 speed road bike, what is the ideal chain tension? 

I was recently told (by a bike shop) that my rear derailleur should be fully extended (pointing forward) when the chain is between the 2 biggest cogs. 

Does a riding with high tension decrease your chain life quicker? It seems logical that it would, so perhaps I was given bad advice.
The chain should be as short as possible and as long as necessary. It will usually shift better with proper tension. The tension shouldn’t affect chain life though the shorter chain will. Make a 104 link chain shorter by 2 links and it has almost 2% less material and thus shorter life. But that’s really a small effect.

I can dimly remember an article which stated that higher tension (they tested big chainring vs. small chainring) is actually more efficient, probably because there are less vibrations.