bottled drinking water has been sterilized, but that doesn't mean its free of minerals, phosphates or nitrates.
YOu need distilled or RO
water ideally for topoffs, and the RO
filters need to still be in good working order, as old membranes won't strip all the nasties out.
minerals don't evaporate. if you top off with hard, mineralized water, you run the risk of building up very high hardness levels in the tank water, which could adversely affect the life inside the tank (corals inverts and fish...the bacteria and live rock will be ok).
Now, having said that, there are a lot of people who use dechlor'd tap water for top off. If you don't have terribly hard water, and are certain it's nitrate and phosphate free, top offs with tap water is a viable option. This can help add alkalinity, calcium, magnesium and other trace elements back into the water, reducing your need to use supplements to accomplish the same thing.
Some will definitely argue tap should never be used. However its hard to agree 100% knowing that SW
aquarists have kept fish and corals alive for decades before the advent of highly filtered water being available to the home hobbyist for reasonable prices.