The "Offset Even" option is turned on by default for the Shrink/Fatten (Alt-S) tool. You can see this by opening user preferences, selecting the input tab and opening the Shrink/Fatten menu under the Mesh menu. However, if you use the tool, you'll see checking or unchecking this default option makes no difference. The tool always defaults to off.
It doesn't seem currently possible, even with custom coding, to have this option on by default. I disabled all transform modal map keys just to be sure they weren't interfering. They are not. I also tried changing the default key to something else (Ctrl-K). It seems nothing can be done to have this tool use "Offset Even" (i.e. even thickness) by default.