Search engines (like Google) see URL which ends with trail slash "/" and URL with default document (default.aspx) at the end of the same URL as two different pages with the same content, title, etc. So for SEO purposes it’s good to get rid of default page at the end of URL. Here is simple rule for URL Rewriter to sole this problem.

<rule name="default.aspx Redirect" stopProcessing="true">
   <match url="^(.*\/)*default\.aspx$" />
   <conditions logicalGrouping="MatchAll">
        <add input="{REQUEST_METHOD}" negate="true" pattern="^POST$" />
   <action type="Redirect" url="{R:1}" redirectType="Permanent"/>

This rule applies for all URLs which have default.aspx in it and makes permanent redirect (with code 301) to URL without it. Next trick is condition for this rule. The issue is that in Web Forms form action and all controls like buttons have page name in it. For the form you can change action on PageLoad event, but not for controls. So condition in the rule is to skip redirection for requests with postback. Next important note is that the rule should be first.