I use NS almost exclusively but am not dogmatic about it noting at least the theoretical advantages of air.  I find the "aspiration test" problematic as the one time I threaded a catheter through the dura I had not been able to aspirate CSF into the catheter.  I depend heavily on the TD with epi and go slow.  As to one poster's note of "losing NS early due to lax tissue in the elderly", my perception is that this occurs in the interspinous ligament which is noticeably less dense than the LF.