i've heard the same thing about sage BUT if you really want to know, send them an email or call them. they have been very helpful in the past for outlining standard thread colors and other things when i built on their blanks.
when fishing for a 100lb tuna with standup rods... are these rods used with guides located on the top? wouldn't any rod with guides on the top twist a bit with considerable load and so, depending on many factors including an anglers skill, location of reel used and other variables, these might attribute to the twist? hold a rod straight out, down a bit or up at an angle. any considerable load (big fish) pulling from below you (in the water) is going to have that affect. the line pulling through the guides will want to move towards the bottom of the rod. isn't there a somewhat recent development by west coast anglers to "acid rod" guide placement to negate "twist"?
read this interesting article (sorry it may seem like an ad but it shows one interesting perspective to rod twist. not once do they mention the spine) grousehunter's mention of fiberglass seems interesting.
What's an Acid Rod?
it seems to me the idea of locating guides on the "correct" spine is a bit of marketing genius. what a great thing to sell to a prospective customer. but i think it's a disservice to those who fish. i still think building on a spine is not necessary. if it makes you feel better to build on a spine read up on the ideas (there are a couple out there) and use one.
for me, once a blank is assembled i'll sight down it. i generally only wrap fly rods from 3wt-7wt. usually the tip is where you might discern a slight bend due to manufacturing processes, if at all. i turn the tip section so that it bends up if that situation exists (we're talking about a slight bend, if it's more, the blank should go back to where it came). the weight of the guides and a line through them when fishing will naturally pull the tip down in a straightening effect. accuracy in casting comes from skill. i leave it at that. i only build for myself, friends and the occasional new friend interested in a rod. if i did it for a living, i would do it the same way until hard scientific fact showed otherwise. but from what i've read, that has yet to surface. even those who have written how-to books on rod building have changed their stance regarding the "need" for locating guides on the spine.