If you look at modern classical guitar tutor books, the majority start with fingers plucking the upper strings 1, 2, and 3 (thumb resting).
I have also seen one or two tutor books that start at the other end with the pupil plucking with thumb on strings 6, 5, and 4 (fingers resting).
There are good arguments in favour of either of the above approaches. Personally I have come to favour starting with the thumb, as
a) it seems to be the easiest digit for my pupils to control – maybe all that practice with texting and X-Boxes has something to do with it? – and b) pupils who start with finger plucking always develop such a desperate urge to lower and flatten the right wrist, and the more notes they learn, the stronger this tendency becomes.
If starting with the thumb, I see advantages in starting on strings 4, 3 and 2 – which is the approach in Rainbow Guitar. Advantage 1 is that these three open strings make a chord – so open string tunes sound melodic and wrong notes do not sound quite so wrong! Advantage 2 is that the right hand is almost obliged to make a good shape and the thumb has to remain clear of the fingers, otherwise plucking string 2 while resting fingers on string 1 is very difficult.
What do you think?