Recently in a game, I was using my new champion mastery as something I would spam after a kill, but I kept getting complaints for BM from the enemy. I asked them how I should instead use it and they had no answers for me. I simply ask if I'm supposed to use this to spam after plays, use to just show my team, or if there are any other ways this is intended to be used.
In my opinion, taunts were made for taunting. Don't listen to the whole BM thing, if you're not making someone feel horrible about their day or something then I think flashing off that you're good at a champion is awesome. ( I always do it :^) )
You use it as a supplement to other spams.
If two pool party 
s have rank 5 mastery, the only things happening in that game are recall spamming and mastery spamming.
I usually spam mine after I miss a skill-shot or make a huge mistake and am about to die. This proves to the enemy and to my team that I intended to do that, so it was worth.
