You can tune the strings any way you wish. There are some factors to consider.
1. Tension on the guitar. The lighter the strings, the less the tension. The lower the pitch, the less the tension. And for you poor six stringers out there, the fewer the strings, the less the tension.
2. Sound taste. Catspaw likes that boom you get with a lower pitch. Some cats don't like it. I believe Kottke actually tunes down one and 1/2 steps to C#. I have put heavy gauge strings on an old 12 and tuned it to B (makes for good transposition for a harp player). Sounds mo' like Lead Belly that'a way.
3. Guitar construction. My 1967 Guild has been at concert pitch for the last 12 years. It's doin' OK. (I use light strings tho').
4. If your tension gets too loose, it is difficult to keep the strings in pitch and to keep from slappin' 'em on the frets as you play.
5. I would recommend strongly against using concert pitch and then tuning up to open E (EBEG#BE)with medium gauge strings unless explosions are a hobby for you and you really didn't want the guitar anyway.
Roger in Baltimore (Just passin' through the Mudcat)