In my case with a Kubuntu 12.04 Live USB, I wonder is it safe ? There have been a lot of updates (150+ or something) since it came out. Specifically with the USB, although I chose the "Discard on shutdown, unless you save them elsewhere" option creating the LiveUSB, is there a chance that something malicious can sneak trough ? Mount my partitions and place something there ? I mean it seems possible to me ( a slightly better noob, that is). And if that were true, shouldn't there be a warning ?
„safe“ is always relative. You are safe from stuff that messed with your operating system. But you are not safe from everything else:
In theory, one could mount your partitions and store data on them. But this data would have to be read again after the next reboot, which would require a different hack.
There are some hacks that write themselves into the master boot record (MBR) of your hard drives, meaning that the attack starts before the operating starts. With a CD, that should be impossible. With the USB stick, I am not too sure.
So you are safer than before, but you are not „safe“ in an absolute sense.
There aren't many risks to browsing from a Live session, at least none that you wouldn't encounter on a normal installation.
If you really care about the security bugs that have been patched since you made your Live USB, you can always run