Hi GC-PAX,
I am posting my experience here. Maybe it will be of help to you. My 485 and EAD were filed in CSC. I lived in Texas. When I went for iEAD since my H-1 was expiring, they rejected it saying they will issue it if and only if the application was filed in TSC. (But, people have got iEADs from Houston)
So, if I needed an iEAD, I had to go to one of the local INS of California Jurisdiction. In case you are in some situation as I was, better go for iEAD now to try your luck. On the other hand, getting it on a date nearing your current EAD expiration date gives you some more time.
Note one more thing here. One of the members on this board had to go after a week or so to collect the iEAD from local INS. If that happens to you, better to keep some slack period before the expiration of your current EAD.
Hope this helps you.