[GMWNashville]

yes it will cause leaks...got my ass kicked on a water leak coming in behind the dash because of that...reason it was hard was because there is cloth batting on top of the paneling under the dash so usually you wont see the water unless there is a ton of it. it will typically leak in by the blower motor...

EDIT: there is no "sticky" stuff under there from the factory..the factory seal is suppose to clip in and sit flush against the windshield...typically issues with this happen when the windshields are replaced the glass guys dont install them properly.