I have never asked ahead of time but I always make sure that the lady knows when I am close. I would never assume that it's OK just because the lady does BBBJ.

I was wondering the etiquette on this...if the girl goes BBBJ and does not say that she does not want CIM, is it assumed that it is okay, or should I always ask first?  Thank you for the help!

makes an ass out of you and me. I never assume she does CIM.  I always ask first.  It's the gentlemanly thing to do.

If they indicate yes, let her know when you are getting close, though she will likely know.  If they indicate BBBJ but no CIM, I would respect her wishes.  If they indicate YMMV, play it by the moment.  What else is there to say?

At least that's what a few gals have told me whose profiles said that they do not do CIM, but did so with me.

So this is very much a YMMV kind of thing.

I think it is good manners to tell them a few seconds before you are about to come so that they can decide what to do.  I think any gal who's been doing this for a while can tell just about when the load is about to shoot anyways.

To improve your odds of getting CIM and even CIMNQNS drink lots of pineapple juice and eat bananas.  Avoid too much meat and garlic.

It does the trick with me.

I have never asked about CIM.  I think it would be very awkward.   She has likely decided before you've gone far if she will allow CIM.   I don't necessarily get a lot of warning but I always try to let her know.   I let her guide the final action so if she does allow CIM, it is in a way she can handle comfortably (without choking).  
skb

If she keeps going you're in luck.

I love men a man is aware his "essence" tastes like his diet. You'd be surprised (maybe not) how many I tell that and I get this face o_o.