I'm not sure what you're asking. Do you mean that you want the Save As dialog to show Plain SVG rather than Inkscape SVG, in the Save As Type field?
No, I guess you'd have to hack the Inkscape code to do that. But it doesn't seem all that time consuming to drop down the menu and choose Plain SVG.
Once you've saved a file as Plain SVG, then the next time you Save the file (using File menu > Save, or the Save button, and not using File menu > Save As) it will save it as Plain SVG. It will keep right on saving whatever the file was originally.
But, if you always use Save As, you'll always have to drop down the menu and choose Plain SVG.