I don't know if it's an "approved" method but I do think today the mob shys away from the technique for a couple of reasons. First, it is a very high profile way of getting rid of someone that attracts a lot of attention; media, law, public outcry, etc... Secondly, the Feds now have some pretty hi tech equipment to identify the origins and finger prints from explosion aftermath evidence. If I am not mistaken, some of the Chicago Mob were identified as assailants in several bombings in the 70's and 80's last year in the big Family Secrets trial. Decades old evidence from bombs was used to help convict some big fish in the Chicago Outfit...I could be wrong but the bottom line is that I think explosives attract too much attention.