IMO, its a bad idea for customers to pick their puppy. Even if you can visit the pups on a regular basis as they grow, any decision would be made on a very, very limited amout of information. This is even more true if you're relying on emails and photos.
The breeder, who presumably is familiar with the bloodlines and parents, is experienced in interpreting puppy behavior, and who lives with the pups day in and day out for several weeks, is the person who should be matching puppies to future owners. No one is going to be able to understand the pup's individual personalities like the breeder. I would be very, very leary of a breeder who lets people pick their own pups. Especially without ever spending time with them.
Yes, you should be concerned with a "reserved" puppy. This isn't a good sign and can indicate a shy personality and weak nerves. Pups should be bold and outgoing.