It's best to train your baby to accept both breast and bottles. If I
remember it correctly, you shouldn't introduce bottles for the first month
to avoid nipple confusion. However, I don't remember the perfect time to
introduce bottles.
I do remember a friend's experience. She had enough breast milk so she never
thought of introducing bottles. Then she had to separate from her daughter
to finish her degree when her daughter was about 6 months. The little girl
just refused to drink from bottles (it could be the taste of formula too).
Her height and weight percentile decreased a lot. She remained to be a poor
eater at 2 years, and she got sick very often. She didn't go to daycare and
her grandma spent a lot of time trying to convince her to eat a little bit
more. I haven't heard about her after that, so I don't know whether she's
doing better.