Taunting is a mind game tool, and yea, it is disrespectful, which is why it works.
General rule of thumb, only use it when you do something really cool. Say you're getting your butt handed to you and you stage an awesome comeback to take the lead-taunt. You pull off a cool combo-taunt. Things like that, nobody minds a taunt.
If you really want to use the taunt to its highest degree, do it when you do something reasonably impressive, but don't get the kill. For instance, whenever I'm sonic, and my opponent's trying to juggle me, if I successfully avoid they attack and land the back air, when I land I throw a "Come on!" (down taunt, rip "you're too show") just to get under their skin. It drives them insane because they see it and want to punish, but back air launches them just far enough that they can't.
Times to not taunt though:
When you just crush someone. You won, don't rub it in too much.
After your opponent makes a mistake. ThIs just shows your an ass.
After a gimp. Unless it's part of a comeback, taunting is very unnecessary. You already got under their skin by gimping them, and taunting takes it too far. You want to display confidence, not make people hate you.
At the beginning. Doing this is just stupid as it gives them an opening, but most people are too chivalrous to take it. It also lets characters charge attacks, so in general taunting is a bad idea at the beginning, unless they do it first-time then it's ok
After you die. This is similar to the last one, but less dangerous. It displays over confidence and makes people turn against you as you've done absolutely nothing to earn that taunt.
After roll/projectile spamming. In general, don't roll/projectile spam at all, and you'll be fine.
And there's probably more, but you should get the idea
