>>Do I still need to apply preventive insect and grub controls?

Insect, no. They'll impact the micro-arthropod population that's trying to help you out by processing those organics for you. Of course, if you develop an insect issue then a targeted response might be necessary.

Grub, maybe. If Japanese beetles or other grubs are an issue for you, by all means do something.

If you want to stay organic, Milky Spore will help with the JB grubs. It can take some years to rise to the level of good protection, so the use of nematodes the first year or two may be necessary.

If you're OK with a chemical, the use of GrubX or the like is fine (I do this as Milky Spore didn't work for me). The stuff targets specifically enough that it doesn't do damage to your insect population and worms are not noticeably impacted.