With DS1 I think I installed the base for our infant seat about a month before I was due. I'm a bit of a control freak and DH isn't all that thorough about reading instructions so I wanted to make sure I installed it while I still could.
This time around I'm not sure what I'll do because I want to move DS1 behind the drivers seat and put DS2 (due in July) behind the passenger. But, I'm not sure DH will be able to drive my car with a seat back there and that is our "family car" (he drives a 2-seater truck with a car seat in the passenger seat). So, installing another seat may have to wait until the last minute. Not my preference though. You know, it just occured to me that I may not install that infant seat at all until after we bring the babe home, since DS1 is still rear-facing and I can just put the infant insert into his True Fit and use that to get the little one home. Hopefully we'll be on our way to replacing the truck by then... if the money gods are on our side...
Pardon my babbling while I just sorted out that issue while typing. I'm glad the question was asked - got me thinking!
ETA: I just realized that my little plan won't work - the seat isn't installed at a 45 degree angle for DS. Back to the drawing board!