This thread is going on 3 years old, and there is some confusion because some are talking about the spring shackle nuts (original question) and some are talking about the spring clips that hold the spring to the crossmember.
The spring shackle nuts can be secured with lock nuts or Loctite if the shackles have no holes. They don't need to be real tight, and depending on how the shackles are made you may need to back off a bit as Tom mentioned in one of the first replies.
The spring clip nuts must be tight and secured with cotters. I tighten these nuts, then go for a short ride over a few bumps, then recheck the tightness and install the cotters.