Lav, my post comes from experience.
Staying in to toxic things can get your Xatu killed, and very baitable, so T-Wave is much better for stopping setup sweepers. T-Waving a Dnite setting up is much safer than switching genesect in directly as you risk being nailed by a fire punch. It also lets you have some form check to a Salamence, because damn if those things don't hurt your team.
Scarfmie may be uncommon, but there is no reason not to be ready for it.
Popping balloon with ninetales sounds all well and good, until you realise you just played against it wrong as popping its balloon just lets it set up rocks. Energy Ball is still a better choice over HP rock, because you can at least hit water types. Hitting Volc sounds cool, till you do the calcs against +1 rona and see that Fireblast hits only 10BP lower, and still hits for crap.
The EVs I listed are just more refined.