It's importance is directly related to the importance you place on your accuracy.

If you don't need more accuracy, you don't need your lies done. If you do, you do.

A modern PW (47°) that has a lie that is 1° out will be off line by 22 feet at 100 yards. So 2° out could be missing the green entirely with a swing that was perfectly fine and properly aligned.

To answer your second question, and upright lie sends the ball to the pull side and a flat lie sends it to the push side. Improper lie angle does not impart fade or draw spin. It dynamically augments face angle. In other words, it doesn't cause a slice or hook, but rather a push or pull.

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True Length Technology Fitter - www.truelengthtechnology.com
It's live! - www.ShipShapeClubs.com
PCS Class 'A' Clubfitter

A new highlight: Golfing the home course on Christmas Day.

I say it too often: If it's golf club shaped, you can play with it.

For the record, I'm a club doctor, not a swing doctor.