Index.htm is not the default page, it is the starter page. The difference being that the starter page is the three frames that hold the toolbar, the navigation pane and the topic pane. The topic pane is what displays the default topic.
If follows that content should not have a topic the filename index.htm as RoboHelp will have to rename one of them.
If you were referring to the starter page, then Colum's blog will tell you more. http://www.cmcandrew.com/robocolumn/archives/2693
I know that Colum has used it without problems but the Adobe line is that it is not supported. I think the reason for that is simply that it has not been tested but I do not know that for certain. Your call.
Is there some reason for objecting to index.htm?
See www.grainge.org for RoboHelp and Authoring tips