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I would agree with you, Frank.
Even though the automatic setting may be convenient, it usually causes more headaches than it solves. I prefer keeping mine set by_request and taking responsibility for making sure the drawing is updated. Honestly once you've run the mass props calculation ONCE, I find I don't have to bother checking to make sure the drawing is updated- it's just done automatically.
Two things I would add to Kshetrabasi's post...
I would move the relation weight=pro_mp_mass into the "Post Regeneration" area of the relations. This way the weight is calculated after any dimensional changes to the model are incorporated. This prevents the dreaded "double-regeneration" problem and insures your weight parameter is always correct. See below...
The second thing is to always make sure you've generated the mass properties report at least once by hand if you're using by_request. In Wildfire 5, go to File->Properties->Mass Properties and select Generate Report. In Creo 2, go to File->Prepare->Model Properties to access the Generate Report button. You must generate the report manually one time before your mass properties values will update.
This sounds like a pain in the neck and a good reason to choose automatic, but I still believe there are more downsides to using automatic than the slight inconvenience of running an initial mass properties report.
Thanks!
-Brian
Brian, Frank,
Can you expand on the problems caused by Automatic? Are they confined to Windchill, or are there some which apply to 'Wild West Mode' 
?
We've recently switched to Automatic, as the only reason we saw previously to avoid it was regen time on complex casings - and with our latest hardware, that's no longer significant.
We have, of course, written a mapkey - but seriously, PTC, who decided that Edit->Setup->Mass Props->Generate Report->OK (hitting pretty much all four corners of the screen) was an acceptable command sequence to update the calculated mass? And why does the Calculate Mass Properties button not do so?
We really, really need a 'rant' emoticon... (and 'flogging a dead horse' - both are available, and widely used, on at least one other forum I read!).
Hi Jonathan...
We have both icons here, too...
Here's the dead horse: 
And the rant:
The rest are here (with instructions on how to use them):http://communities.ptc.com/blogs/SomeGifs/2010/09/28/animated-gifs
But to answer your question... automatic slows things down. If you have multiple models without a density, you're constantly prompted to enter the density before a regeneration can proceed. It's just a bit of a pain in the neck. It's just like auto_regen_views in drawing mode. Yes, it's convenient to have the system automatically update all the drawing views... but for experienced users the time hit you take for such automatic features isn't worth it. This is the same for mass properties.
As far as I know, if you leave automatic turned on, you don't need to do the "Generate Report" thing. That's only if you're leaving it on by_request. At least I think that's how it works. You can tell I'm being a bit lazy today otherwise I'd have fully investigated the whole thing before I posted anything.
Thanks!
-Brian
Excellent - I'd edit my post to add an icon in, except apparently I can't. Thanks! 
Anyway... if the slow-down is the only hit then I think that's fine for us. Anything without a density needs fixing anyway (in fact, our problem is usually black-hole density of 1 tonne/mm^3) so that should only get you once... after that you should have given the offending parts a suitable material.
And yes, the Generate Report mapkey was for when we had By Request set. We seemed to need to use it every time the model changed, though, which got a bit annoying when you made a change via a drawing dimension and then had to open the part anyway just to update the mass.
