Depends on which sweet spot you're talking about. On the snare, if you're looking for a bit of ring, the sweet spot would be just a bit to the outside of dead center. Usually with tuning though, you'd want to not only hit the dead center to check for tone, but you also want to tap directly in front of each lug to make sure there's even tension across the head.
You know, it just takes trial & error. I have a set of tones in my head that I go by and achieve on whatever kit I play. Then again, I try to adopt a "no-touch" policy on kits other than my own. As in "don't change a thing" unless it's a necessity.
As far as that "tune this drum to this pitch" thing that DW uses, I cannot agree with that. There's too many intaqngibles that stop that theory from working 100%. Besides, I like my toms tuned slightly outside of that "perfect long pitch" that would denote "perfect" shell/head resonance.
Tune for your own ears, not from the "manufacturer's suggested pitch".