Should I reboot after an update?

[Damien Hull]

I'm running an Ubuntu Dapper server. Should I reboot after an update 
(upgrade)?

I've asked this question on IRC. I was told by one person to reboot. 
However, others have told me that rebooting is only required when you 
get a new kernel. This brings me to my next question/problem.

I've seen "linux-server" in the list of packages that are going to be 
updated. I can't remember if my kernel ever got updated though. I have 
more then one server and "uname -a" says I've got kernel 
2.6.15-23-server on all of them.

Does the kernel ever get updated on Ubuntu server?
Should I reboot just to be safe?