Actually timers need to be nulled. As said all agents. Exception is only if you never remove/destroy the object, so there is no sense in nulling it.
Players will be never removed. -> no sense to null players
A static trigger gets usually never destroyed. -> no sense to null it. (if you really never destroy it)
With not nulling timers... You probably mean if you use the library TimerUtils. Then you don't really need to null the timers, too.
The reason for it is simple that the system will re-use the timer instead of destroying/creating it each time.
If you use timers just normaly, you also have to null them.