After the first four weeks, I started pumping because I was told by a lactation consultant to give my LO a bottle once a night. I did this, my son had no problem with taking a bottle. BUT I think that pumping caused an oversupply of milk. Many people told me to just bring baby to breast and not worry about pumping so early. I really wanted to have a supply of milk just in case I needed to go out. WELL, I have too much milk, and my LO would choke, gag, we had to work on this! Many of my friends who have babies wanted to breast feed but thought they would have success just by pumping. Needless to say, they stopped BF by 2 months. It is very important to continue to bring baby to the breast, this will stimulate and increase milk supply. The pump can not replace or replicate the stimulation from your child's mouth.
My LO is 4 1/2 months and my milk supply is pretty normal now. I pumped the other day, first time in 2 months! I bring my LO with me every where and BF him everywhere! BTW, I hate cleaning bottles, pump etc.. so much work when all I have to do is unstrap my bra. BF is soooo much easier.
I am going back to work in 2 weeks so I will need to start pumping soon. However, I am not too worried because I will pump at work and my LO will still be fed from the breast most of the time.