If many versions of the same page can exist, you can use the canonical tag to set the one really to use.
So, if you have both blabla(dot)bla/index.htm and blabla(dot)bla/ indexed, you can set one as being the one to use.
Disclaimer : My posts on SEO are just from my observations and I do not say it is a true fact... A real fact of life is that, I'm often wrong...
Here's a good article on when to implement it and when not: 5 Key Rel="Canonical" Implementation Considerations
In your case, I'd say there's no need but read the article for more insight!
Everything will be ok in the end
If it's not ok, it's not the end
Interesting article Ann. Like many sites, I added canonical tags to all my pages as a "it can't hurt" strategy, even though I have a redirect set up and WMT preferred page set up.
For these kind of on-page issues I look at the source code of several household name sites to see what they are doing. Recently I saw that Apple was using canonical tags to point a page at itself. Looking today, I no longer see the tags. Many major sites use the tag to point at itself. According to the article, Bing may have an issue with this. So, I'm on the fence as to keeping or ditching the tags. Technically they serve no purpose if you have addressed canonical issues at a higher level.