You do not want to leave the throttle wide open, this will flood the cylinders with fuel. Doing so has the potential to wash the oil out of the rings and thus throw the readings off. That is more of a concern the more cylinders the engine has.
What is acceptable really varies, while most folks figure an engine is an engine, there are many different configurations as to how the internals are built and some are more stringent as to what compression range is acceptable including the variations between readings. The manufacturers specs should be adhered to versus a "rule of thumb"
Agreed with broadhopper a leakdown test is a good idea as it tells you more than just a compression test.
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