If you say you don't care, then I think you are not being a responsible member of society. You should care and you it is easy to check, so why not do it.
No, it's not. After the thief uses a coin mixing service, there's no way to tell where those coins come from.
It is not easy.
Even without a mixing service, there is no way to prevent somebody from sending you tainted coins.
Furthermore, what level of taint are you willing to accept? All coins other than newly mined ones will eventually have 100% taint as coins are stolen over time.