Thanks Ken for clarifying. I should have been more explicit that the difference in a Daaa and Dadd bridge is VERY slight, and will likely equate to only a couple cents difference. It is nowhere near enough difference to sound "off" however, if you are keen on developing your ear to hear subtle differences over time, in my opinion its good to have the most precise intonation possible.

Thank you both for the information. 

Unless you have perfect pitch, for all intents and purposes, any ordinary dulcimer like a McSppaden can be played in either DAd or DAA without any issues, simply by changing the tuning. You'll nevere hear the difference.   Any ordinary dulcimer can be played in any tuning, provided the strings are appropriate for the tuning you want.  A standard string set for DAA/DAd will let you tune up to FCC or FCf before the strings get too tight; or down to BEE or BEb before the strings get sloppy loose.

There are indeed dulcimers built specifically for certain tunings.  But McSpadden is talking about tuning the dulcimer a certain way, not building it to be only played in DAA versus DAd...  Nate is right to say that the intonation can be tweaked slightly, but most people can't hear the difference...

 
The key difference between a dulcimer indended for Dadd or Daaa is the intonation at the bridge. On a dulcimer intended for Dadd, the melody side of the bridge will be VERY slightly closer to the nut than on on a dulcimer intended to be tuned Daaa. This is because in a pack of strings intended for Dadd, the melody string will be thinner than the middle string (eg 22,14,12,12) whereas in a pack intended for Daaa, the melody string will be the same gauge as the middle string.(eg 22,14,14,14) The thicker a string gauge, the further away from the nut the bridge needs to be for the string to be properly intonated.
So basically, if you play the instrument in both tunings, you should pick whichever one you play more often. The bridge placement will be slightly better intonated to that tuning than the other. Most of the tunings that you mentioned are closer to Dadd than to Daaa, so if you play them all regularly, I would recommend the Dadd instrument. 
Nate

The dulcimer I currently have is one I purchased back in the mid-1990s before we even had internet at my house.  I walked into our local music store that rents school band instruments (bought my oboe for band through them).  I asked about buying a dulcimer and they pulled out a catalog of instruments they can order. I think I paid just over $100 for it.  No maker marks on it.  Basically if I wanted a dulcimer, this is the one I had to buy.

I want another dulcimer, and pretty much have settled on a McSpadden standard 28.5" hourglass.  I see one of the options is having it tuned to D-A-AA vs standard D-A-dd.  Which tuning should I get, or does it really matter?  I play in both D-A-AA, D-A-dd, also in D-G-cc, D-A-cc and D-G-dd.  There are other tunings in some books of Celtic music I have that use CGC, CFC.