When I did extract I tended to just go from the tap right into the fermentor with the wort. I can't say that I ever had a bad batch because of it. But it certainly can happen. If you have a way to boil water for 15 minutes, then store it in a sanitary and sealed contain while it cools back down to a useable temperature...that is the safer way to go.
If you are pitching enough yeast then its probably not a huge concern as you are getting started. And if this is your first batch I bet your LHBS has given you one packet of yeast or a tube of yeast and that's it. WHICH IS FINE FOR NOW, don't fret over the amount of yeast to pitch yet.
I say to just go for it unless your water comes from a well or something. Municipal tap water should be pretty clean. If you pick up a contaminant its likely happening right at the faucet, not from in the line. Just be sure you don't have dirty dishes and sponges splashing around in the sink as you get the water for top off.
I think you'll be fine from the tap until you are ready to move up a little bit in techniques and process.