Depending on your level of paranoia, a single-pass overwrite (srm -rfs) may be good enough. It would thwart any software recovery utility, and you've gotta wonder if your data is worth so much to someone that they would hire a pro to open up the drive and piece it back together...

Chances are that when running srm on a journalling file system (like newer installations of HFS+ with journalling turned on) it won't matter how many passes you make. The only difference would be time (there is little-to-no added security value). So, if you have journalling turned on, but still want to use srm, I see little reason not to use -s.

Of course, I have not verified that OS X's srm doesn't somehow magically account for all the implications of journalling, but I would be surprised if it does. Can anyone speak more intelligently on this?