Little info on not cutting things off that I have figured out with help from various parts of this site. Basically, what talyns said, but in more detail, and I think something should be added to the base tutorial to the effect.
Basically lets assume you have a screensize of 1024x768, fairly common size, and a 3/4 aspect ratio.
lets say you make a viewport of the same ratio, for example you want a little room on the bottom and side of the screen and set your viewport to /viewport 0 0 824 618. Nothing will be cut off, the entire view thats visible with 0 0 1024 768 will be visible, ie the rat at the bottom of the screen will still be visible. everything is just a little smaller.
Now if you decide you are just looking for room for a textbox along the bottom, and try /viewport 0 0 1024 618 to give just as much room at the bottom, the aspect ratio is wrong, and therefore the screen is basically the same as 0 0 1024 768 with the bottom bit of the screen cut off, ie you wont see the rat in front of you. everything is the same size as normal.
From my persective, I tried to viewport over a year ago, and didnt realize this, and gave up on viewports until recently when I realized that aspect ratio is all important.
So, in simple terms, keep the aspect ratio right, or it wont work the way you probably want.
ps, you can for example go with something like 100 75 924 543 to keep the window in the middle and give room all around, the only rule of the aspect ratio is that the resulting window needs to be the same aspect ratio as your view mode (usually 4/3).
Heh, hope this helps anyone who looks in here