lucy24 - 8:50 am on Aug 18, 2012 (gmt 0)
strikes me as overkill. You said at the beginning that your index files may have php OR html OR htm. That's three. If someone asks for a nonexistent extension, go ahead and dump a 404 on 'em. And if someone asks for "index" without extension, they are probably up to no good. So cut it back to
And then, once you've canonicalized all your directory names, you can rename the physical index files so Apache doesn't have to waste time looking for three possible files every time there's a request for a directory. You might possibly need both php and htm(l) but you can definitely regularize to either html or htm throughout. The Apache default is html, so that will save the usual nano-micro-thingie.
as long as your server is receiving requests for index.html you will need a redirect to the canonical url.
If any of those requests are coming from the googlebot, you can condense that sentence to "forever" ;)