Yes, I agree with Balder, I would use the equal house system to be safe. Sometimes Koch, or Campanus or one of the less used (at least for natal work) such as Regiomontanus or Topocentric may work further north than Placidus but most of them break down eventually - at the North Pole it is possible to have the same sign both rising and at the MC, so Houses systems that use the Ascendant as the Cusp of the first and the MC as the cusp of the tenth just won't work.
Equal Houses work under all conditions as long as an Ascendant can be established. What's more it is much simpler to cope with under such conditions and recent work by Project Hindsight tends to show that the Equal House system has a respectable pedigree and is not simply a device for Astrological novices.
I tend to use Placidus houses for most of my work - I 'feel' that the MC should be the tenth House cusp - when it breaks down I use Equal Houses or occasionally, if it works, Koch Houses.
Minderwiz