I recently noticed that one of my computers was connecting to an SMP/DP across a T1 to a remote office to store user state during OSD. After reviewing the logs I saw that it was connecting to the local DP for content, but not for user state storage.
I couldn't find anything in the logs that explained this behavior, and I checked my boundaries and boundary groups and everything looked good. After failing to resolve this on my own over a few days, I opened a ticket with Microsoft support.
It turns out that while using boundaries to connect a client to the local DP is a feature, connecting to the SMP at the local site is not and the client can connect to any writable SMP, even if its across your slowest WAN connection to a remote office.
You have two options to avoid having the client connect across the WAN for user state storage; check "Enable restore-only mode" in your State Migration Point properties under Administration > Servers and Site System Roles, or check "Capture locally by using links instead of copying files" under the "Capture User Files and Settings" step in your OSD.
Enabling restore-only mode will prevent the SMP from being used to store user state, and user state already stored on the SMP is still available. The problem with this approach is that not only will your other sites not use that SMP to store user state, the local clients won't use it either so they will be sending user state across the WAN to another SMP. You may have to enable or disable this setting as needed, for example if you are imaging multiple workstations in the same office you could enable this setting on all other SMP's except the one at the site where computers are being imaged.
Capturing locally by using links instead of copying files would prevent you from having to remember to change the SMP settings frequently as it would be a "set it and forget it" solution which is what I prefer.