It doens't matter what the height is. Keith horne played his bass so high it barely touched his neck. Zakk wylde played guitar so low yet shreaded lighting fast licks and crazy harmonics.

If the guitar is too low, you can't(or hardly) use your pinky to press the high notes on the low E string. That will hurt your left wrist a lot ! Lots of guitarists who put their guitar low oftern uses their 1st 3 fingers to play solo. Unless they have long pinky fingers like paul gilbert ~! :eek:

However I saw lots of instructional videos played by sessionists, they go for a variety of tunes. That is they put their guitar near their chest so that they reach all the notes on their guitar.

For me I played the guitar like John Petrucci did. If I wanna do some high notes solos, I will push the guitar more to the left of my body making it more vertical. After all I have short arms and short fingers. :p