the height doesn't really matter! i ride with a breastcollar every time and yet i only jump 2"3 to 2"6
I use a breast collar almost every time I ride because my saddle slips back but with them you have to make sure it's not affecting their shoulder movement
As long as your saddle isn't sliding back, and your horse isn't tossing their head, you don't really need either. In the jumper ring a lot of people just use them for looks.
I agree with Jessica it doesn't matter the height it just matters whether you feel you need it or not.
If your horse likes to flip his head then a martingale can be used at any height. I personally use a breastplate to keep my saddle in place when jumping. I don't think it really matters the height as long as your tack is doing it's purpose. Hope this helps!