I print 24"x30" on an Epson 7600, and find that 33% crop is good for
setting unsharp mask parameters, as its close to actual printed size.
My recommendation is that you make a print, and try to match your
screen size with the print size.  
Peace,
-chasfs
http://chasfs.com

Unless you're making large enlargements, it tends to expose flaws that
may not be noticeable to anyone in, say, a 4x6 print.

--
Todd H.  
http://www.toddh.net/

I think you have a severe tunnel vision problem if you need to get back 12
to 15 feet to see something that is only 38 inches wide.  Perhaps it's time
to see an eye doctor.

100% pixel to pixel is the best way to evaluate the photos as there is no
pixel aliasing then and it does go to show just how well digital images
enlarge and how much you can get away with when you shrink it down to 6x4"
for a print too.  Noise has to be really bad to be noticeable at 6x4".
Small amounts of mis-focusing, or even a lens with poor sharpness get hidden
too when the image is reduced so far.

However, your monitor probably has less than 100 pixels per inch (my 20"
1600x1200 LCD is about 101 pixels per inch).  This is only the resolution of
a fax machine on coarse mode.  Prints usually can show up to about 300
pixels per inch so you do need to put some care into them as those with very
good visual acuity can see the 0.003" details in the prints so keep viewing
at 100% to check.

It really depends upon how much you plan to enlarge, and what pixel
dimensions your camera produces.
If you only print to 8x10, but you have a 16MP Canon 1DsMII...then it is
pretty pointless to check at 100%, since you'll see tons more blur than what
will show up in print.  Another example:  100% view of my 6MP Canon 10D is
going to look more "in focus" than 100% view of (for example) the new 10MP
400D, simply because you're taking the same shot and essentially elarging
any focus errors that may exist.

So in a nutshell...  -There's no "rule" about 100% for focus checking.  You
basically have to learn to judge what is acceptable to you for your intended
use and enlargement needs.

It's useful to check for sharpness and detail, and also for editing of
fine detail when using LCD monitors.

In practical terms, it's generally a non-issue. It is useful in tests
and evaluations of lenses and cameras to check for critical performance
issues.

But the kind that a "numbers freak" who plays that game would be using
as quoted information to backup their reasons why camera N is better
than camera C, is a waste of time. I get a kick out of the twits who
post 200-400% crops and point out miniscule differences and then
proclaim Canon better than Nikon or vice versa.

I'm more interested in taking pictures than arguing insignificant
details.

You are not trying to see the whole picture. All you are looking for is
adjusting sharpness. If you try to correct for sharpness and noise at
anything other than 100% crop you may well make matters worse.

For sharpness evaluation and other similar things [i.e. noise], 100% is
essential so that you see pixels as pixels ... otherwise you do not know if
the issue is the image or the rendering of the image on your monitor.  100% is
not a crop by the way ... at 100% image size, you usually have to scroll the
image around to evaluation the areas you would like to look at.  If I do any
work with the healing brush in Photoshop, it is always at 100%.

I use 100% for culling & sorting & evaluating sharpness & noise. I print
at 13x19" which even at 10MP is a significant enlargement to print at
300dpi. But at 300dpi original size pixels (about 8x10 for most DSLRs, 6
or 10MP isn't that different) the print will be softer than what you see
on the screen so expect things to be a bit softer. When you start
enlarging prints or cropping, it's critical to zoom even beyond 100%
because CRT computer monitors are not made of perfect square
even-colored pixels. I also zoom way in when doing RAW conversion
because sometimes ugly things happen in the process and if I'm bothering
to use RAW, I usually want it as good as I can get.