How important is balancing a generator?  I know its important, but what ratios are we talking about?

I just had a transfer switch with dual wattage meters installed (for my 7000 watt genset)

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IIRC, generatorjoe.com recommends keeping the legs within 1000 watts of each other.  I occasionally turn on light circuits and try to keep mine within 500 watts.

It's fine taking bursts over 1000 watts when electric motors kick in.

Balancing is primarily done to insure that you don't have a bunch of high draw devices only on one leg which could burn up the winding or constantly trip the breaker in the generator.  

A rare occurrence from having the load unbalanced is when one side of generator but not the other creates torsional vibrations in the rotor that could result in broken shafts.  It's not often encountered but it's easy to prevent through more precise balancing of the circuits from the watt meters.  Just remember that if you help out a neighbor and allow them to run an extension cord you won't see the wattage they're using because it's not running into the transfer switch and through the load meters.