I would also tend toward a bit extra per person, especially if it is in the evening...
Are you serving some accompaniments? Nuts, fruit (figs, grapes, apples, etc.), quince for the manchego :-), bread AND crackers?
I might also think about how much wine people will consume and if there is driving involved after... That is the soon to be mom in me talking... :-)
1/4 pound generally sounds about right - I agree that it depends on the time of day and whether people will be hungry.
I have found that if my crowd is more conservative, I need more mild cheeses, and if the crowd is adventurous, I can play and get more robust varieties.
I also tend towards more food but almost always have tons of cheese leftover.... Maybe we should start a thread about how to use up all that leftover Manchego....