Are mechanical scales accurate?

[airlane1979]

I have a cheap scale which varies the weight by ±5kg depending on how I lean forwards or sideways, which is pretty useless. Are all mechanical scales like this? Do electronic ones also have this variation? I don't want to replace it and find that they all do it.

[MissCandyGirl]

I don't like weighing myself: it makes me miserable and depressed. I often hope it is the scales that are wrong: rather than me.