I do full boils, so I don't usually top off after the boil. I prefer using all RO (reverse osmosis) filtered water for brewing. I know this filters a lot of the good minerals out, but I have a salt-type water softener with chlorinated city water, and I have bad luck brewing with the stuff straight from the unfiltered tap. I add a little yeast nutrient to the RO water, and everything takes off just as it should.