anything below 7 is good for spot free drying.
edit
sorry, your asking if this is a normal process. Well not all water that comes from your tap will be the same tds - so expect fluctuations. Dont forget the speed at which water is filtered though media (whatever you use) also effects the filtration process.
Depending on your locality, tds will differ and so the higher the tds the quicker you will need to replace your membranes and resin.