[texhax] The Em Rule, a Question of Style

[P. R. Stanley]

Hi folks,
I read somewhere that the em rule should preferably be prefixed with 
a non-breakable space character ( ~ ), mainly for stylistic reasons, 
however, there was no mention of a ~ suffix to ensure that a line 
break doesn't occur immediately after the em dash.  Would a 
conscientious typesetter place a ~ on both sides of the em dash, or a 
prefix ~ is the only stylistic requirement? Further, what would the 
list think of the idea of using \mbox to make sure that a line break 
doesn't appear on the left or either side of the em rule/

Thanks
Paul