I do make an effort to use woods with similar radial shrinkage percentages (if they are quatersawn, only the radial will matter), but it doesn't seem to be critical. For one, any back will shrink in low humidity, and if the joint is backed up with a cross grain strip the failure line will not be on the joint (it's likely to be alongside the reinforcing strip). Three piece backs are not really any worse off here than two piece: dry them enough, and they will crack somewhere. For another, once woods have dried, they don't take up as much moisture for a given humidity as they had at that humidity when first drying, so the real picture for shrinkage and expansion of cured wood is less than the shrinkage tables (which are from green to fully dry, usually kin dry) would indicate.
About the only risk I see if the woods had very different shrinkage percentages would be rippling or cupping, etc. And one just doesn't see much of that if dry, seasoned woods are used--even with rosewood/maple combinations.
I agree that it will often look better if the center is a bookmatch, so that the back is actually four piece. Depends on what you have to work with. Here's one now in progress with a one-piece center. Bookmatching the center wasn't an option; I made the center from an orphaned board. African blackwood and cocobolo, about ready for sanding.
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Howard Klepper
http://www.klepperguitars.comWhen all else fails, clean the shop.