IMO Springs don't need breaking in. It's actually you that needs breaking in, you need to get the technique right before you can get more height. People just blame lack of technique on their springs not being broken in.
I can back this up as when I moved onto my second pair of stilts I could instantly jump as high (and run faster) as I could before, the springs did not need breaking in at all. This has also happened to other people I know who have moved onto a second pair of stilts.