Sometimes a torso bomb just can't be avoided, and it may even lend some flavor to the 'Mech; I know some of my own designs have that as one of their potential Achilles' heels.
That said, explosion risks aside, one problem with all the ammo in one location is that once that location goes, even if nothing blows up, all your ammo-dependent weapons still on what's left of the 'Mech stop working. That's part of why I like to distribute my bins a bit and, if possible and necessary, CASE both side torsos instead of only one; I also like to stick my ammo close to the weapons it's feeding both as a matter of personal taste and so that said weapons can provide a bit of padding.
I virtually never put ammo bins in the legs, and if I do it's usually on quads because I need that torso space for the guns (and because, once a side torso goes, I lose the corresponding foreleg anyway). On biped chassis, especially one with a standard engine, I'd rather lose both side torsos than one leg; and remember, in close combat leg bombs are a standing invitation to both kicks and infantry.
