I am tier 5 and just test it. You can go at tier 1-2-3-4 dungeons. BUT you can't go at multi-stage dungeons at lower tier than yours, except if you have storyline quest to go there. And you can't attack players from lower tiers at those dungeons.
If you really have problem visiting lower tier dungeons (and wasn't multi-stage) post again and i will move the topic to bugs.