Depends on the size of your tank and perhaps the delicacy of your species. As tanks get larger and the percentage changed gets smaller, its less likely the cold water will cause much downward temp movement once it mixes in. Many professionals swear by shooting in cold water.
On the other hand, our advice to beginners here is to always consider playing it safe. Take your time with these controversial things, do the safe choice and your day for forming an opinion based on your experience will come!
The "safe" choice is to always roughly match the water temperature of the incoming water with that of your tank. Using the skin of your hand to roughly judge this is fine, its close enough. Also, unless you have some reason to think your hot water system leaches too much stuff into the water, most of us just use mixed hot & cold right at the tap to get this "roughly matched" temperature.
Personally, the way I do it currently is as follows: I route the output of my gravel cleaning siphon into a bucket in the tub (so that I can catch any zebra danios that get "exploration happy!) After I've gravel cleaned and about 40% of the water has washed through that bucket and out the drain, I play with my plants and do algae cleaning and any other things that are more easy with the tank water level low. Then I take a big plastic cup and take a cupful of that tank water out of the bucket over to the sink with me where I'm going to hook the Python to the faucet. I adjust the hot/cold of the faucet until my hand says it matches the water in my plastic cup. My faucet has a single level for temp and it stays where its just been set while I turn the faucet off and screw on the siphon hose. Then I turn the flow on gently and check that its going into the tank ok. I splash in half of the Prime dechlor and finish filling the tank, the splash in the other half. The details don't matter though, just get the temp roughly matched. Having it flow in a little bit chilly is something the fish usually like, so minor mess-ups and variations are not a big deal.
Please note that I'm talking average tropical tank beginner here. Be aware that there are people who have very expensive fish and people who prefer to be very safe and precise mixing their conditioner and their kettle-heated hot water into buckets prior to the tank add, and there's nothing wrong with that, its even safer and it may be right for you for some reason.
~~waterdrop~~
