I never expressed until Ella was around 4 months old and it was only then because I was going back to work part-time in a couple of months time and wanted to get her used to a bottle. It was a bit of work getting her to take a bottle, but she took it at 6 months without any problem and now she will quite happily take the bottle or b/f.
If you are happy b/feeding and have no real reason to introduce a bottle, then I would personally hold off until you are ready- no rush.