PHP, Apache are big ones if you have cPanel it's just a script run to do it. But not doing these depending on the nature of the exploits may cause you to be compromised in some way. MySQL not so much but it's always good to keep with the times. Also to improve your security you generally upgrade your kernel. The majority of the defaces on web hosts is just one script exploited then the kernel is out of date and there is an exploit to gain basically full write access. So all it would take is you not updating one of your sites scripts and getting exploited or even custom made scripts getting exploited and you could end up with your entire server being messed up.