The Apple iOS 7 although called as minimalistic, has got a lot of hidden features (check iOS 7 features) which can be unfolded only if a bit exploring is done. Same is with the timestamp checking in the iMessages app. In the messages conversation, the time stamp is shown above the message only if at that time, a thread of few conversations happened.
There’s something you can do to check the timestamp of each message in the conversation with a particular person. It’s not that Apple iOS 7 doesn’t store the time details of every message, but it keeps the thread simple by hiding that, and you can check that if you wish to.
To do that, you have to open the Messages app (or the iMessages which it is called), and open a conversation with any contact where there messages already present. In that thread, swipe your fingers from right to left to expose the timestamp, which would appear on the right side.
Just like you can open the timestamp by swiping from right to left, the swiping in opposite direction would again hide the timestamp and bring back the normal conversation which appears by default.
It would be a time display for each message in the conversation, which was actually the same in the previous iOS versions, but now as Apple has started making things simpler and better, we are getting to see a lot of hidden features which are not totally removed but are hidden and can appear only when you want to see them.
As of now it is still the iOS 7 Beta 3 version that is available for the developers to download and try out, and its a long wait already for the Beta 4 version to be released by the iOS makers. You can download iOS 7 beta 3 for iPhone which works with the iPod touch latest gen too, and the iOS 7 Beta 3 for the iPad and the iPad Mini.