Can't say I have ever had pads that seize too the pistons so never felt the need to apply it there.I've used copper grease on brakes for years and will continue unless I find a good reason not to.
Only a thin smear on the parts of the backing plate that are against the piston or calipers.
A cotton bud helps get some on the little tabs that slide inwards in the caliper as the brakes wear.
I've seen Ed China plastering the back of pads with copper grease on Wheeler Dealers.
A good reason to stop using copper grease for me would be pretty much all manufacturers advising against it's use. It can interfere with some ABS setups.
Ceramic grease is just much more suitable for the job.