I trim mine before I shave... but can not say that there is any reason why I do it that way
I trim mine after, and I also don't have any explanation as to why I do it in that order.
I think its because trimming the goatee is an afterthought. I only remember to do the goatee and sideburns about every 10 shaves.
I use corded electric clippers with a #2 bladeguard ... and its taken me far too many years to discover that I get much better results trimming the goatee ATG, using S-N strokes. Any other direction and it comes out looking ragged and unkempt, especially the moustache portion.