I'm going to the contrary position here, supported by both training and experience.

• I was told in a luthiery course I took that strings sound best when closest to their breaking point. Tuned higher. Now of course I don't like breaking strings, but I break very few in standard A440 tuning (or DADGAD A440).
  
• Sometimes, like when I have new strings on and have tuned without a reference, I find the guitar (or fiddle, or mando) is sounding in tune and OK, but only OK. When I get around to checking it, and bring it up to concert, it suddenly sounds GREAT. So I believe from experience the first point above.
  
• As Chris C noted, when you tune low, "Don't expect the intonation to be great." In other words, you'll be, um, out of tune if you go anywhere up the neck. If you are Stevie Ray or Jimi, you could make a lot of micro-corrections on the fly, but if you don't have that acute sensitivity to pitch, you're just out of tune.   
  
• And slack strings sound flabby and tend to go more out of tune.
  
• Playing with a capo on increases tuning problems, especially if you bend strings much.
  
  

Now of course there are times when the most important thing is to avoid stressing the instrument--as with a 12-string or a vintage guitar with no truss rod--or you really want a different kind of sound. Fine. But mostly, I'll stick to concert.

I have a personal conflict--I like to play in DADGAD, and I like to sing in C. The two are not compatible unless you tune a whole tone down from concert. I guess that's one of the reasons I'm a two-guitar-lugger, one for standard, one DADGAD.

Play on regardless.
Willie-O