centers are generally in the middle, half way down and half way across.  if someone's managed to do an eye map where that's not true, but it's a single eye, it's kind of nuts.  but i was thinking more about general UV usage, and things needing to start somewhere different, rather than just, say, using a U or V node raw.  i wouldn't have to know any co-ordinates to, say, create a gradient on most faces.  i would if i needed to know the top was 1.5 instead of 0. i have used U and V nodes for various tricks before, and since i often have to keep other numbers in my head to remember to apply or transpose or whatever among materials, i personally and individually don't really want more of them.  that said, i think i'm not your average user. 

D|S, on the other hand, is a big deal.  its users are plentiful and vociferous.

and just to say, because i don't think it comes out in text, "if you can" isn't a challenge.  it's just recognizing that seam guides take time to make and you may not have that time.