Right I was wondering, how many dice do you use for your rattling guns? I always use 3 because using 2 is reliable and un-skaven :D

3 is the safest bet, as 4 is too risky. I roll 2 dice always, and if i dont roll a 6 and a 5, I roll a third die. And if you roll 1-2-3 on your first 3 dice, why not roll another one? You can get more hits in, but you MAY get a 1 or a 2, which isnt really that awful, and you MAY get another 3, which is somewhat annoying, but still not a big problem. The upside is you may get 9-11 hits if you dont misfire, which is ahellofalot better than measly 5.

Yes,the one showed by Slai is the best way to use ratlings i think...

i'll role 2 if don't roll a 5 or a 6 i'll rol another one

You see, that really depends. For example:
I fire against chaos knights who I have set up a trap for. It don't matter really if I cause a casulty, but it is still nice. I choose 3 dices.

But if I fire against a small fast cavalry unit, and start rolling a 6, I roll one more dice. Then I stop, because I don't want to risk a misfire.