Mechanically it will not hurt your gun. You, on the other hand, are a different story.
As was mentioned shotguns lack the whizbang firing-pin lock that is commonly found on handguns. I know the safety on the M4s blocks the sear, but this is not good enough for me, and I don't know for certain that the M2 does as well.
I do not recommend keeping a shotgun "locked and cocked". I don't care how you look at it, there is a hammer under stress backed up to a shell ready to go BOOM.
A few nights ago I went for a walk in the woods. I promptly slipped on some mud and busted my hiney. Goggles, Beretta and Mossberg went EVERYWHERE. Accidents happen, and better safe than sorry.