Should be fine, its better if you can get the water close to the temperature of the tank, but its not totally necessary. How many gallons is your tank?
sorry to be critical but it is very important to get it up to temperature before you put the fresh water in. sudden temperature changes stresses fish and also leaves them open to things like whitespot. i run a tap for 5-10 mins with a themometer under it until it gets exactly the same as my tank. then i fill my barrel and keep the thermometer under the running water. if i have enough time i run the water the day before and leave a heater in the barrell over night to get it perfect. and make sure u treat it correctly. if you have carbon in your filter that will help if you donot get the correct de-clorinator mesurements. hope this has helped.
In smaller tanks it is more critical, however on larger tanks where the water volume is higher, the temperature change is going to be less, which is why I asked about his tank size.
For my 60 litre tank I do a 25% water change once a week without fail. When topping the tank back up I fill a bowl from the tap (with the aqua safe) and then add a little water from a boiled kettle. I swish it round with my finger until the water feels a similar temperature to the tank water. Then pour it evenly and slowly across the top of the tank to spread the fresh water and avoid disturbing plants, substrate etc.
u get small bits of lime scale in ur tank that way which isnt healthy for fish. when i have time i get ro water from lfs and leave heater in that. good luck