I once asked my friendly engineer that question and he told me that the sight glass should be full when the tail is right down to the ground, so that is what I do. the oil is usually so thick when it is cold that that is the only way you can see it anyway.
Problem is that if your pasenger gets in because it is cold standing waiting, the tail is very heavy to pull down!
It also checks that the skids are not frozen to the ground as happened recently in a UK accident.