In general I would advice to update and if you can not for whatever reason to try to keep up with at least the security updates. Those tend to fix flaws that might compromise your system.
The risk? For a personal system those seem negligible to me. The amount of problems reported where a Ubuntu desktop is compromised seems close to 0% compared to Windows.
- You might run into a bug that was already solved in one of the updates at a time where it is inconvenient (bugs always appear when you least need to see them).
- You might want to update at some point but due to kernel updates you need to reboot several times to get things updated.
You might get into trouble if someone decides to specifically target you and you have security holes (in flash, a browser or your kernel as examples).
Better safe than sorry. Do try to update and always make backups of your personal files.