I've heard that the primer used is just to protect the panels in transit and may be just the cheapest available and therefore the non-etch type which is preferable.
For outside visible body panels, I'd certainly remove the primer unless I was sure it was an etch-primer, rather that risk problems later on after a costly respray.
For floor panels, I'd try power wire brushing the primer to test it's adhesive powers, if it seemed securely bonded, I'd leave it alone - it would be quite time consuming/awkward to strip primer from floor panels.
If you do decide to strip the primer, I'd avoid sanding, instead would opt to wipe over using paint stripper. It might be possible to remove certain types of primer using an appropriate paint thinners.
Al.