Okay, I'm going to confuse things here -
My excellent and pro-BF MW told me that (big disclaimer here) as long as latch was good, there were no weight gain problems, baby was generally feeding okay from the breast, then it was really important to introduce a bottle BEFORE 6 weeks. If you wanted to have the option of bottle feeding later on, that is.
There's a narrow window when they take to it easily, but once they have got used to breastfeeding they can be very reluctant, or just refuse, to take a bottle, whether it's expressed milk or formula.
She also said to give a bottle every couple of days, or once a day if convenient, and NOT to offer the bottle once, think, oh, DC is fine with that, and then not bother again for 2 months. Because they aren't going to remember how to do it then.
I asked about nipple confusion - she said for one bottle a day or less, for a baby with a good latch and no other problems, it wouldn't be an issue.
This was at just under 2 weeks with DS. It worked totally fine for him, he's 8 months now and has never had formula, just BM, largely from the boob! But having the freedom to give him the odd bottle has been SO helpful.
I know women are told to wait... but there are so many posts on this board from women who are desperate for their 4/6/8 month old to take a bottle, normally because they have to go back to work. They waited, and their baby didn't want to know.
There are other issues around this, for the mother, to do with supply and expressing, but I just wanted to share that.