Going to 6 bolt rotors for the first time since I have always used centre locks. Should I use threadlock when installing the bolts if the bolts dont already have it on?

I never have – I use Coppaslip – & have never had one come loose

if you're buying new the bolts will already have them on. I've always used it and never had a bolt come loose or had trouble removing one.

Dont use threadlock. If you want to lock the bolts just put some silicone on the threads.

I've always put threadlock on rotor bolts. That's how they came & so I figured I'd keep up the tradition.

Not sure if it would make any difference if you put a dab of grease on & used the right torque with your torque wrench.

After having a rotor bolt pick up the threads in the hub on removal, I always use some grease on them, there are 6 bolts, and 3 is more than enough, use all 6 with a dob of grease on and you wont have any trouble.

Most come with threadlock so stick with that.If you check over the bike often and are happy to not use threadlock then do so.from a workshop side of things – you cant expect everyone to always keep on top of their bike so its threadloc all the way.
if the fixing holes are bored straight through, be careful to not put on too much/forget to wipe off excess from the other side. It can drip down into the braking surface.

Never quite understood this one – to undo / soften stubborn threadlock you use a little bit of heat.

I know disc bolts often come with pre-applied threadlock, but don't brake discs get hot?…..

threadloc them just is case, make sure both surfaces are clean and grease free for it to bond, contact or brake cleaner best to use. Greasing a threadloced bolt will stop or reduce the effectiveness of the threadloc.
A bit paranoid in know but better to do it right.

use some low strength threadlock like Loctites 222, will stop them from vibrating loose but they will still unscrew fine…

Everyone overtightens rotor bolts. that's when threadlock causes issues. 6nm is less than a wrists worth!
I've happily used spraypaint when I couldn't find threadlock.

That should be 6Nm before the SI unit pedants come and get me.