i voted for hot tap water, i have a near new combi boiler and mains water pipe is pvc and then copper about 12" before it enters the boiler, then 12" when it comes out then its onto flexi as the sink is about 24" away from the boiler, as inchworm said i let it run for a minute or so first to clear old water and anything that may have settled there, the only decent lengths of copper pipe in the house go to the radiators, and i sure ain't going to get my water from there!
i think as long as you use common sense with any new system its safe, with older houses id think twice, old copper pipes will seep more into the water, lead pipes a definite no no, and also if you have boilers or pipes fitted in the last year or so then they now fit pipes with an anti-fungicide on the interior of the pipes on a regular basis which has been known to wipe out tanks. a few things to think about if you don't know your boilers and pipes too well!
just thought id add, i don't use hot water from the tap to try and match it to my tanks temperature, but i do 50% water changes, if i filled 50% of my 1000 litre tank with cold tap water i think id definitely be saying goodbye to some expensive fish, and on the same note it would take a bloody lot of kettle fill ups to warm 500 litres of cold water!
i suppose if you have an average sized tank and 20-25% water changes then yes you'd be fine just topping up with cold water.