All through out the design and slice/coding phase I try to play around with different sizes by simply resizing my browser window down..
The main problem I'm starting to discover with many sites now, is they aren't thinking about all of the widescreen users out there, therefore using a 100% stretching method seems to look pretty wack most of the time... then on the flip side, make sure not to design a design for some major widescreen display, forgetting about all normal display users ;)
There is such a thing as min-width and max-width (and conditional statements for IE6) you know. A very wide page presents readability problems as the user has trouble finding the beginning of the next line when it is 19" away, and 5 - 7" is supposed to be the optimum. I think that we should avoid having lines of text more than about 9 - 10" in length on our sites.
On the odd occasions I've used 1400px wide monitors or wider still ones, I've usually had the browser open at half the screen width. That's very useful when comparing how two browsers display things, for example, even on my 1200px wide screen.
Why is my browser not full screen? Because I've just this instant seen the email show a new one arrive as its display just changed. Or I'm copying some stuff from a Word doc to a web page, or using a database and checking that the online one matches the offline data. Lots of times I have FF and IE running side by side, especially when troubleshooting a design.