I always assume its part of a punt anyway.
In over 10 yrs of punting I've never been refused it or charged extra for it.
It may put the girl off if you try and jab 2 fingers straight in but if you warm her up with a gentle pussy massage first then gradually slip one finger in then (if she's wet enough) slip a couple more in.
Works for me every time 
Wow, in ten years you've never been refused fingering once?
Sorry to be cynical, but I do find that very hard to believe. As I definitely can't make the same claim...
Anyway, along with making sure my nails are trimmed properly and preparing the area first (usually with my tongue, sometimes by massaging with my hands), if things are unclear I will ask permission first. In my experience, some will say yes, some will say no. Easier IMO to just have that question answered in black and white before the punt... I'll ask if I have to, but I don't really want to have that conversation with a prossie mid punt, or pre-punt, as it can either be a bit of a mood killer or a yes OK go ahead kind of thing. It does tend to be my experience that those prossies who do have their preferences on this explained on their profile already, I will tend to have a better time with them, whether the answer is yes or no. So to me, this should, in theory, be a good thing.
I do hope it doesn't become an extra thing though. I can already hear the Roma's warning up their "u wan fingerin baybee? Is estraaa baybee! Dont u wanna thinger me baybee" etc lines.
