I'm firmly of the assessment that Sheldon is an INTJ. Having spent *some* time in physics grad school, and given how incredibly well this show portrays actual physics cultures, I'd like to point out that there are VERY few S's in physics grad school. Further, the ones that are generally prefer to be in the lab and DOING things, or tinkering with things, NOT sitting around and being all theoretical and whatnot. Think "look mommy, I made my own laser!!!", not "given moderate assumptions, one can show that the odd-dimensional topology of quantum-loop vortexes as expressed in its deRham invariant is clearly a violation of CP invariance at large enough scales for long enough time periods, assuming adiabatic processes of course" [yes, thats a made up sentence, but with lots of actually-valid words that just don't belong together, as far as I know].
Anyways, Sheldon is a *theorist* NOT an *experimentalist*. I'm trying to remember, but isn't he a string-theorist to boot??? That's right, super-theoretical AND super-mathematical [and if I'm misremembering string-theorist, whatever he does IS super-mathematical]. I've met plenty of smart S people in technical fields. None of them had an interest in theoretical mathematics, or theoretical physics for that matter.
Also, Sheldon is pretty damned physically oblivious and physically unskilled [I don't want to drive a car!], Ni-dom tends to be extreme senso-tards [or at least the ones in physics generally do]. Remember the episode in season 1 where he wants to built something, but has no tools, and then they find a toolbox, and he doens't know how to open it!!! That's hardly S.
Having spent *some* time in physics grad school, having had many peers, and having known many people [including professors!] with PhD's in physics, which the character Sheldon has, the disgustingly vast majority are INTJ's, and I've NEVER met a S physics PhD, ever, period.
Boy genius is INTJ as snot. And some INTJ's are pretty darn persnickety about details and HOW things will be done. That doesn't make them S's however.
Plus, hello people, when he daydreams about time machines, and having his own virtual city [Sheldonopolois he calls it], and what he will do [and won't do!] when he is the last human being left alive, or if aliens come and try to abduct him, and ..... and and all the other wacky crazy-as-shit stuff he thinks about and then when he tells people your reaction is "WTF??? who comes out with weird shit like that, and who has time to think about stuff like that???" Ni.