Kiwitrees has not yet been tested properly on 5.6, and I don’t currently have an ability to test or support any issues you might come across, so I advise against it for now.
Jamie reported an issue with the Zend library (http://kiwitrees.net/forums/topic/new-translations-for-kiwitrees-3-1/#post-4603) , but I don’t know if that’s a set up problem at her end or something more significant for kiwitrees as a whole.
Once I release 3.1 later this week I will be upgrading my test environment to PHP 5.6 and will ensure that 3.2 is compatible with that.
Of course if you want to ‘guinea pig’ kiwitrees on PHP 5.6 for me that would be great 🙂 The downside of a small community such as we have here is a lack of testers. But I feel that is more than balanced by the friendliness we have here.