topping up coolant normal on bmws?

[omodos]

What is the truth here?some people say it is normal to top up bmws, and others say the cooling system is highly pressurized and should not need topping up and does not lose coolant...

Does it self vent?if you top up too much for example?and when does the low coolant warning light appear for those of you who have topped up when they got the message?is it indeed when the the bob float in the coolant reservoir hit MIN level?or did you lose much more coolant than that.

I have notice that when I topped up my 3.0d 2004 e53 that taking bob/rod levele from MIN to MAX level hasn't taken more than a cupfull of wate/coolant mix......