Well, it depends what your recipe is. If your recipe is for a 5.0 gallon batch and you have 4.5 gallons post-boil, then topping it up with .5 gallons would water it down compared to 4.5g, but would bring it up to what the recipe calls for. Otherwise, your wort would have too high of a concentration of sugars, which would result in a higher ABV when the product is finish - potentially throwing off some of the balance in the recipe. You ultimately want the batch size at the time you pitch your yeast to equal the batch size of the recipe as intended. Going back to your first question, if you know how much water you typically boil off over the course of your brew session, there is nothing wrong with your approach of starting out with more water so that you end up with exactly the quantity of wort that was planned.