[ygra]

They shouldn't really matter, except for file size. They won't magically use different SVG markup. All Plain SVG does is strip away Inkscape's XML namespaces and custom attributes/elements. There's a fair number of SVG features that are poorly-supported, broken or with a number of gotchas in various renderers, however and you're kinda expected to work around those yourself.

For icons and similar you can often go back to the most basic things: Filled paths with solid colors and gradients. That should render pretty much everywhere where there's at least some SVG support.