I hear fly rod guide spacing/placement isn't as critical and important as spinning/casting rods. Is there any truth to this and what would the reasons be to why? Thanks!

Absolutely no truth. Guides perform similar functions regardless of blank type. It's just as important to evenly distribute the load forces on a fly rod as it is on a bait cast or spinning rod.

Who told you such a thing?

Most likely someone was referring to the aspect of casting. On a fly rod the line isn't being pulled off the reel spool by the cast, but rather by your hand prior to the cast. So this mitigates some of the work in locating the butt guide insofar as casting is concerned. Beyond that, however, it's exact as Donald said above. A rod and the related stresses are going to be the same regardless of rod type.

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on fly rods,I have always used the idea of loading the rod, and looking for flat spots created in the line, relative to the flex of the blank. Moving guides to avoid flat spots so the line follows the curve of the blank. It is important.