Interesting question because I am learning to tig pipe and the guys I am learning from mostly pass the filler rod through the inner diameter up to the root opening and fill that way. It makes for nice xray quality welds when view from the inside or out. this was on SS, chrome and the inconel pipes.

it depends on what the client wants. to much "push through" can be bad. sometimes you want to melt the wire in and other times throw it on the inside for a fat root bead.

Learn to keyhole from the outside, backfeed from the inside and just walk over the wire as you'll get the opportunity to have to use each technique eventually.

Hi Toddler
interesting question, I generally do both on the same pipe if in the 5G(horz axis) position
my understanding is this
unless it is done because of lack of access
generally the placement of the filler wire is all about allowing for gravity
after all, weld metal is a liquid when it goes in
so at the bottom of the pipe you add the wire inside (above) the joint so when i falls due to gravity it does not end up concave as you work up the sides the wire should be level with the root and when you get to the top it should be outside (above) the joint so when it falls due to gravity it is not convex or excess
so to answer your question it depends on the position you are welding in
2G or Vertical axis sucks back all the way around on the root so generally operators add it inside the joint to compensate for the suck back 
Hope this helps
R
Flash
www.technoweld.com.au