My Alvarez 12 string is now about 9 years old and I have always kept it in standard tuning. I will say that it has developed a noticeable hump in the top at the bridge which has not affected the playing but does not go away when its de-tuned. I think if I had it do do again, I might detune and capo up to prevent this.
Have fun. I think the longer sustain achieved by the harmonic of the paired strings gives a 12 string a very interesting effect when finger picked or flat picked. Just arpeggiating the chords gives a nice rolling, ringing sound compared to a 6.
But remember the saying about 12 strings... "12 string players spend half the time tuning, and the other half out of tune." ;>)