Hi CB,
I've got both the 10th and 19th Editions.
Although I tend to use the 19th more, I've found that there are some things not covered in the 19th, that are in the 10th, so I've held on to both of them.
As far as I can tell, they both cover just about everything you are likely to need routinely in normal work but I guess as newer 'technology' comes along, the older stuff gets slipped out of the latest Editions. On this basis, I think any version would be pretty useful but strangely enough, the older versions might just have some info, that whilst long obsolete in Industry could be very useful to a Model Engineer (as Rod says).
I guess a fairly recent Edition would be a good choice but you certainly don't need the latest/newest one. I didn't pay a lot for either of my two copies, so I'd probably look out for one in a S/H bookshop (19th) or Charity Shop (10th) and be guided by the price.
I also use the Tubal Cain reference book very regularly although there is nothing like the depth of detail as there is in a Machinery's Handbook. My TC is well thumbed (e.g. black!) in certain areas and for simple things (drill tapping sizes etc) it tends to be the first thing I turn to on the bookshelf when I can't find the Zeus Tables anywhere (because i've put it down and the little man has moved it!!)
Regards,
IanT