It's not real good on your neck or truss rod to go from real low tunings to standard tunings a lot. It's alright to do it everynow and then. But make sure you're not changing back and forth numerous times in one day. The constant tension and then lack of tension can cause stress fractures in the grain of the wood... tiny little cracks that you may not be able to see... unless you have a thick coat of lacquer or paint... these cracks will allow moisture a one way ticket in... Then comes the warpage. If the cracks are exposed to too much humidity they can get bigger and even lead to broken necks... Obviously this is the extreme case... But just use caution when tuning your guitars to really low tuning then back up to high tunings... Especially if you're using heavy guage strings... the added tension just makes it that much worse.