It depends on the amount of resources available on your hardware.
QlikView Server performance has improved in the last few years, but my organization still restarts the server nightly to ensure there is no residual memory issues. We are regularly running close to capacity on RAM, so it is more of an insurance policy.
If you have an established maintenance window, then it would make sense to schedule a restart for that time. This might simplify the implementation of OS patches that require a reboot.
Hope this helps,