All depends if you can find the 9mm you specifically bought for the job but on the day can't find itPeople spend a gizzillion quid on wheels, spoilers, seats……and then grind a 10mm tool down to 9mm to work on their brakes!!!!!!!!!
Not worth doing even with a dividing head and a surface grinder.
I always put a tiny film of anti seize on the disc / hub mating surface given previous experience of having to cut off discs that rusted to the hub.It’s nothing to do with upgrading the brakes, more a quick check, especially important with used discs which could be warped. If you get some dirt under the mating surface between disc and hub it will also cause axial runout.
You can definitely get away without a DTI on new discs providing you are careful with cleanliness, removing rust and any burrs. Don’t put grease on the mating faces.
If you spin the wheel after bedding the brakes in and get intermittent dragging it indicates that the disc is not running true.
Same applies to alloy wheel/ disc mating face.