Just my practical opinion (I don't have a daughter, but I have had lots of tampons):
I don't think my mother showing me how to use a tampon would've helped me any. A mother's vagina is very different to a young girl's, and it's not like there's an obvious knack that she'll see when you do it, as there's a lot of internal manoeuvring going on. If you wanted to try anyway and you're physically open with your daughter already, I don't suppose seeing her mother do that would harm a well-adjusted girl too much. If you don't routinely go in for that level of openness, she may find it alarming. It would certainly be an image that would've stayed with me, I think!
I thought I'd do my good deed for the day and come up with a more helpful suggestion. Anyway, it turns out a Google search for
tampon "how to insert" returns a very helpful set of links, one of which is a link to Tampax's website containg a helpful instruction video (cartoon diagram, hence clean and has internal view). I've just watched it myself (can't hurt to brush up on technique

) and if she can't follow that and the accompanying page, she's probably not ready for tampons.
http://www.beinggirl.com/en_US/teen/older/pages/p_tampax_howto.jhtml[/url]