Even when I take it and feel like I am answering totally different from I have in the past.
I always try to reject any association I have with what the question is measuring, and try to focus on what the question is asking. I imagine best and worst case scenarios, then try to measure which way I lean. When I am taking these tests, I am usually running an estimation of my score. Despite feeling very much that I am going to come up as some other type than INFJ, I always answer as honestly as I can. I actually find it interesting that I am going to come up as something else, like ENTP or INTJ because I am answering yes to a lot of Ti or Te oriented questions, and in a way I am kind of hoping that I will, as my honesty will have paid off if I do. However, I almost always come up as INFJ... very rarely coming up as barely INFP or barely ENFJ.
But... I also acknowledge that despite my honesty, I'm only inputting my self image, and that my self image has probably been swayed subconsciously by knowing how I am 'supposed' to be, as I've taken the descriptions to heart. THAT is the real danger of self assessment, identifying with a type. It can cloud your self image, which is really the only thing a self assessment test can measure.
When we take self assessment tests, we're only getting an indication of our self image. Knowing about the types may be as detrimental to a true indication of type as not understanding the questions.
Personally, I don't think types exist, other than to describe common patterns in cognitive function preferences. People are actually nothing more than the summation of their own unique cognitive function sets, which are constantly growing and adapting. If anything, the only tests that have relevence to me are the cognitive function tests, especially considering that the axis approach that the MBTI type tests use is an extremely assumptive way to imply function preferences.
I do have to admit however, that it is interesting that on MBTI type tests I usually come up INFJ, sometimes INFP or ENFJ, and in rare cases ENFP. On cognitive function tests I almost always come up Ni ≥ Fi ≥ Fe > Ne. There is clearly a synergy to these two approaches, which either means that I'm testing rather accurately, or at minimum I have a clear sense of self (which may or may not be entirely accurate).