The number of ranks and the width of ranks for warriors quite varies from playstyle to playstyle.
Ofcourse there's no question as to what you want them to do, the answer is kill, maim, burn,....hmm perhaps going a bit to far into the future. Anyway, the question is, are the expendable to you or not? Do you mind losing them after a few rounds of combat or not?
It also depends on what weapons you give them. As you could see 3x6 or 3x7 is the most chosen option as this maximizes attacks, allows for a shooting/magic casulty or two and still hold ranks. I used to play 6x3 as well, three blocks of nurgle with shields. Now they are great tank units and can deal out quite a punch. But I found out that str 4 doesn't always do the trick, especially not against more elite troops. So the things you need to ask yourself are, is it worth it to maximize attacks with the equipment i have? Can I afford to lose them or not? (this might influence your decision of maximizing attacks to get more CR, or to get more ranks (so 5x4) to get your steadfast.)
5x4 is effective with shields, great tanking unit with good save, steadfast, still a steady 16 attacks, 21 with frenzy banner.
6x3 works good when having a sorcerer or anything inside it, more protection cause more warriors have a chance of obliterating the unit. Also best way to maximize attacks. Units with FC tend to go up to 350pts though.
7x3 is a personal choice, with 8th edition other armies also put larger units on the field, also ranked on 6 or 7. Just don't forget your base is 25x25 and the others mostly 20x20 (considering the more popular armies now). And it wouldn't be the first time, I've had a fussy opponent about every 0.001 inch (as a matter of speech, even though they are right, it's annoying like hell). Though this is an absolute maximize of attacks.
Everything higher is a waste up to horde rules, then it get interesting again.
To give an idea, I field 2 10man units of khorne warriors with halberds. I can afford to lose them (but I rather do some damage first). They are a small target so will never be much shot or magic'd at. Worth around 200 points and hit with 20 attacks I5 WS5 at str 5 at their best, good enough against most armies (except for those pesky high elves).
So that's what I have to say about it, it's not really a direct answer but one thing I learned about warhammer is that lots of things are personal choice, when you feel good with it, you go for it. So I just gave you some info, the playing and testing is up to you. Enjoy it, and ofc glory for the dark gods