AbleCommerce Shopping Cart Software

I have to change to "On" because of McAfee :

<customErrors defaultRedirect="Default.aspx" mode="On" />

I noticed there is no redirect page in the default webconfig page for AC7.
Is there any reason why I shouldn't add one?

Yes you can do this, this feature will be available in next version of AC. But it would be better that instead of using defaultRedirect="Default.aspx" you must create another page on the root let say it ErrorPage.aspx and then use that page like because on this new error page you will tell the customer that some error has occurred.