Ok so full disclosure, I have always been a boob man.  Love em.  All sizes, natural preferred but a well done set of manmades are awesome.  Some years ago I was dating this girl (civie) who had some bat shit crazy tendencies but loved sex and had a really nice rack.  Not only that, but she had the most amazing gum drop nipples.  Awesome!!!  All these years later I still dream about them.  Whenever we had sex she totally wanted me licking and sucking those nips.  Certainly worked for me!  Turns out that her boobs, specifically her nipples seemed to be wired to her clit.  She could literally cum if you licked and sucked her nipples just right.  Consequently I was all over those beauties.  But like I said, she was BSC about a bunch of stuff so the relationship did not last.  Since then, and ever before, I never met a girl whose nips were wired to her orgasm box.  Nothing like it ever since.

How else to account for the fact that some gals will hardly let you touch them, and others seem to want you to bite them right off?

Have been with several ladies who really loved their nipples sucked. A certain provider I know really loves to have her nipples pulled, rather hard.

I had a Korean Hooker GF once upon a time who could only cum one way. She would sit me upright in the bed so we were literally face to face while riding me in a modified CG position, and then she would have me squeeze and twist her nips HARD while she would the shit out of me. It took me a while before I could bring myself to twist her nips hard enough to satisfy her as any other woman would have been brought to tears (or would have punched me) if I twisted her nips that hard. but that's what it took to push her "over the edge".

I've had a few.  
One of them actually preferred nipple play and clit stimulus to anything else.

Occasionally my wife (on the very rare occasions when we have sex) will orgasm from nothing more than licking, nibbling and sucking on her tits during foreplay.  Its not every time and definitely not during certain times of the month when they are more sensitive due to her cycle.
She loves "normal" clit play oral as well but rolling a nip between two fingers while licking drives her absolutely nuts.  Put 2 fingers of the other hand inside and pump with a come hither motion while licking and she's bucking out 4, 5 or even 6 O's before we even get to my cock being inside her.  She finishes off with one or two more before the end.

I had the same experience of the same situation. Many years ago, I dated a girl for a couple years who would have orgasms through licking and sucking her nipples. Hers were a natural C cup. No contact with clit was needed and this was for sure real with all the tell-tell signs of authentic orgasm. I will never forget this phenomena. We would play with her hands tied behind her back in a chair so she couldn't touch herself and while playing so nicely with only her nipples she would drool very, very heavily from her puss to the carpeted floor. I would quickly lay underneath with my mouth open for the clear sweet honey drooling. The carpet would be like we spilled a whole bottle of foamy KY, but it was all from her.  

... when I gave her shiatsu style massage on a specific point on the back of her legs, where the glutes meet the hamstrings in the middle of each leg. I've had other women tell me they like massage on that spot, but never Oed from it.  

For a moment, I thought this was going to be truly entertaining and unusual.

Rubbing her boobs or nipples during DATY, that's my signal that she's one of those ladies that has a connection between her nips and her clit, so I take over and experiment a little until I find out how much stimulation she likes. It varies from girl to girl. Some just like a little bit and others get me concerned I will permanently pull their boobs into a pointed cone shape. Once you find her "zone" it can cut the O time from DATY by half.  However, for some women, they don't like any simultaneous nipple play during DATY at all, and find it more if an irritation than a stimulation.

and I thoroughly enjoyed stimulating both

Told me from day one that her nips and clit were connected, and her reactions to having both stimulated (I love doing that!) orally or with my c&ck confirmed what she told me.  Most of the other ladies I've seen in my lifetime, providers and civvies, have been more or less as such.  Always pay attention to the girls...only good can from it!  

Good for you....research away.......Even better, conduct your research like a government study, you will be insured no conclusion to your work.

And I got this one from my wife, that large breasted women's nipples tend to not be as sensitive (hers were not).

Licking, sucking, and even pinching or biting with light or moderate intensity are all great. And of course I've got appropriate kinky implements such as clamps and pumps to enhance nipple play.