M.C.
It might decrease your supply for a little while, but your body would quickly pick up to meet the demand. (Holding baby while pumping will help stimulate production.)
The only problem with pumping after feedings (for storage... It's great for building supply!) is that all milk is not the same... The milk at the beginning if the feeding (fore milk) is thinner, meant to sate the immediate hunger. The milk at the end (hind milk) is thicker, and gives the baby more staying power and delivers more calories. So pumping at the end will give you bottles of hind milk, and if your body is over-supplying for those sessions then baby gets full on fore milk before even getting to the hind milk. Adding a session in the middle would work better for storage purposes as you will be getting both hind and fore milk together. :) pumping after nursing also tends to increase your supply in a weird way... Instead of slowly building up, it would be like BAM! Boobs are full! And they would be FULL. To the point that I had a forceful letdown, and my daughter would choke trying to nurse if I didn't wait for the milk to quit shooting out (much to the entertainment of my husband, and anger of my child...) before letting her latch on. :/
So I would really recommend the middle-ground pumping, based on my own experience and the composition of the milk... BUT, pumping after nursing is an option that works fairly well for many people, (maybe I just have funky boobs? ;) lol.) and is definitely better than nothing if pumping in between doesn't work for you.