This is quite a basic idea that a lot of self-taught players get wrong.
Basically, we want to hold our guitar like a classical guitar player holds it – the guitar sits on the same leg as the fret hand at an angle, so the tuner pegs are roughly in line with our eyes. There are several benefits to holding the guitar this way. This position is the closest to the position the guitar is in if we are standing up and playing. Also, it allows us to keep our back upright and untwisted. If you look at the typical “rock” way of holding a guitar when sitting down, where the guitar is on the same leg as the picking hand (not the fretting hand), then the back has to be slightly twisted in order to hold the instrument. This can cause long term problems with your back – definitely something we want to avoid!
Here are some photos to demonstrate the ideal way to hold a guitar when sitting down:
Here is the incorrect way, the typical “rock” way to hold the guitar when sitting:
And here is the correct way, guitar on the fret-hand leg, tuning pegs roughly in line with my eyes:
This is just one of the many things we teach and train our students on, to make sure they become the best guitar players they can possibly be.