Nightly builds are generally stable, but since they are made nightly, sometimes testing does not find all the problems. The release is considered a stable build as well, but as with all software, there will be bugs, some discovered, some not.
If you report a problem that we have fixed we will indicate that your should try the nightly build. It is, of course, up to you to decide if fixing the problem is worth moving off the release build. All fixes we make are in context of the nightly builds. There is a much higher probablilty of a fix breaking the software if it is applied in isolation than for the nightly build to be unstable.
Even with new releases you should consider testing for your environment before you go live. Who knows, maybe a bug specific to your environment exists so no one else has seen it. (Another good reason to test nightly builds--then you have a better chance to have `your` problems fixed in the general release.)