I think my regret when we build will be not having a big enough budget to get everything i'd like.
We added a rear roller door to our garage and it cost $774.
We wanted to have better access to the back yard for landscaping etc as well as being able to park a trailer out the back rather than leaving it out the front or taking up room in the garage.
If you want it cheaper, we asked them to put the T-bar up and leave it open, and we'll get a roller door added later. It cost $70 to get the back open (as we'll have a garage door at the front so it doesn't matter too much to keep it open at the back for a little while) and it'll cost heaps less to put the garage door in afterwards then what the builder quoted. Or you could do what my parents did - left the back open and just stuck in a double gate. It keeps the dogs and kids in, but in hot weather the air flows through the garage when the garage door is open, so it's not too hot if you're in there vacuuming and so on. Does get a bit dirtier though.