Test your tap for nitrates and phosphates and ideally copper before using. If the levels are 0 measured with a standard aquarium kit I would use it.
Also is it treated with chloramines? If so I would consider it a no go.
As for top off, use distilled or RO
water. But occasional or emergency tap water top off is ok.
Don't forget to treat the water first with a salt mix that contains detoxifiers or with prime.
As for water changes the principle is the replacement water as to be cleaner than the tank water... So measure levels in the tank and your tap water ( they can change throughout the year) often to make sure water changes are actually helping.
I am using tap for water change and managed to improve my tank nitrates and phosphate levels significantly over the last few months.