Ahh, the twelve string tuning question.

First, any stress on the bridge is going to deform the top. Tuning a step down will just take it longer. It will not prevent the top from bellying.

Here is what I do with my old 12.

I installed a bridge doctor right off the bat. Not a plug here, but it did flatten the top and I think there is a slight improvement in tone due to the top correction. There is definitely an improvement in action. Hey for $30, you can't go wrong on an old 12.

I tune to concert pitch and leave it that way, unless I'm not going to play it for a week or two. Then I tune it way down, still leaving tension in the strings, but way down.

my 2 cents.

Nick