I know in my own training that you often fail to exhibit a fundamental principle every so often and produce a poor technique (such as extension, centre etc). I don't think I could put one above the other - I like to review what I seem to be lacking at the time. However if there is one thing that puts you off a technique it is trying to concentrate on several things at once. Ideally I hope to practise the principles which I am weak at (and therefore need to be aware of these principles) but a good technique is performed when I have forgotten all about them and it comes naturally. So possibly relaxation/openess could justify as my key principle!