Now that is an excellent question Don.Personally,for me,it depends what it is.Notches in a keel for the frames,I would aim for exact width of the frame but would leave a little meat for height adjustment.On Saint Philippe's frame mortices I go for exact depth but tweak the length with a sharp Scalpel.I try to minimise how much I allow for adjustment otherwise you can loose the sharp shape produced by the mill
Kind Regards
Nigel