Technically, no you shouldn't as you need the water to be hot enough to kill any bacteria in the powder when you add it to the water. That's the official line on making up feeds.
In reality it depends on your comfort levels. My kids were on a reflux formula that had to be added to cold water or it wouldn't mix and we didn't have any problems. Freshly boiled water put straight into a freshly sterilised bottle and sealed straight away is considered sterile for 24 hours. I don't know what the percentage of babies being made ill from bacteria in formula pwder is, but if you consider the number of bottles of formula made every day in the UK and the number of babies you know of that have been ill from formula (I don't know of any) I don't think the risk is that high, but it does exist hence the guidelines.
If you sterilise properly, boil your water fresh every time and don't keep the bottles hanging around too long then *I* think it's fine to add the powder, warm and feed straight away. The other thing you can do is to fill a flask when you go to bed, that should still be hot in the morning but again may not be hot enough to kill bacteria. Bottles of water and adding powder is also the easiest option for being out and about apart from the sealed ready made cartons.
HTH!
Laura