I was just wondering if it's a bad idea to have the width of the core layout containers to vary in type?
Assuming that your main wrapping div is set as a percentage width would really be the only condition when percentage widths would be effective on the child divs to keep them in relation to the wrapper's width.
Yes, you could set two divs at 50% wide in a wrapping div that was 800px but they are not going to change so they may as well have been 400px to begin with. However I don't think that is what you are asking.
In a situation where you have a percentage width set on the wrapper and then defined pixel widths set on two sidebars for example that can be done without causing any problems too.
Here is an Example where the main center content is given the flexibility to fill the available space between the sidebars.
The layout is confined to min-max widths so it keeps it usable without the fixed width sidebars ever sitting right beside each other.
Bottom line is that you need to control it someway so the layout does not break when the space gets limited.