You gotta take the whole arrangement into account. if you insist on the hard-panned lead vocals, then you gotta look at where you want to put everything else, not just backing vocals. As Dani says, there are no rules saying that "this has to go there" specifically, but you do want to look at which tracks are in danger of masking, muddying or otherwise fighting each other because they share similar sonic characteristics and/or arrangement roles.
For example, if you have a call and response kind of thing happening between backing vocals and another instrument, you may want the vocals a bit to one aide and the responing instrument a bit to the other. If the accompanying vocals are more of a choral instrument than a lyrical vocal, then you want to ask yourself whether you want them as a more localized track or with a more beneral background spread to them (either way works, it's to taste). Any way you slice it, you'll want to keep spectral balance in mind, giving each instrument each room to breathe on it's own (e.g. if the vocals and the guitars or the keyboards clash, then keep them away from each other. If they are complimentary sounding together, them move them close to make more room elsewhere for the rest of the stuff. etc.)
G.