All my Seagulls have taken the changes in tuning quite nicely, from standard, DADGAD, Open G and Open C mostly... I've never even expereinces a 'setteling in' period...

Regarding multiple guitars, I thingk that while it's handier to have a guitar for each tuning, it is in no way a harad fast rule that you HAVE to... Watch a GOD like Don Ross, Or Stephen Fearing play a show on one guitar and be changing tunings all night... No setteling in period there either... and no excessive wear and tear on the guitar or the strings...