There are times in the bush where blindly following POH take off distance calculations will kill you. The chart does not anticipate the soft section which will drag you down just before lift off or the hump in the strip (% slope is an average over the whole distance). There are strips in PNG that have parts of the strip at around 20% slope even though the average slope might 'only' be at 10-15% for example.
I don't think there is a single bush pilot that hasn't had a nervous moment on take off or landing. If not, he/she has not been in the bush long enough...