| Of course this depends on whether the server you administrate is accessible only by machines you trust. If other machines can connect to it then you have to stay aware of any possible attacks from outside.
Whenever I log in on servers that are connected to the Internet, I always check the logfiles for suspicious activities and take the proper actions.
I frequently read security reports of the software that is installed on the server and have subscribed to the security mailing list of my distributor. | 
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