This is what my MW told me a couple of weeks ago -
For babies who are latching on well, generally feeding happily and doing well BF-ing, introducing the odd bottle feed shouldn't cause nipple confusion. For babies who are struggling a bit at the breast anyway, and have trouble latching on, she wouldn't recommend it.
She also said (and this interests me a lot, given the number of posts on this board from mums struggling to get their older BF babies to take a bottle) -
There's an 'ideal' window of time, before the baby is 6 weeks old, to introduce a bottle. Later than that, and they can really struggle to use a teat, or just refuse it altogether.
She also said that if BF-ing mums introduced a bottle of expressed breast milk successfully at this stage, then it was best to keep it up once every 24 hours or so. Just that it wasn't realistic to introduce a bottle to a 4 week old baby - think, oh, great, she feeds fine with it - then not do it again for weeks, and expect baby to 'remember' how to do it.
I'm no expert,just relaying what MW said, and sharing my experience. Following her advice has worked perfectly for us so far - DS has been taking 2 night time bottle feeds from DP since he was 2 weeks old, very happily, with no trouble switching between bottle and breast.
Having the break and often up to 5 (FIVE!) hours of PROPER SLEEP has made BF-ing so much easier for me. DP enjoys it, and DS is doing very well. (I should say, I find expressing v easy, which I know is lucky, and not everyone does).