activating a phone on a custom rom?

[sevenspeed]

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If you flash a custom precooked rom before you activate a phone, does that prevent the activation codes from working like the mdn?
I gave my mogul to my brother and he couldn't get it to activate. I tried myself but ended up relocking and reflashing with stock sprint.
I did get it working but just wondered about that or if something else was going on. I tried searching but kept finding threads about gps activation.
If it does prevent proper activation maybe a note could be added in the unlock stickies for others that may do that, I spent quite some time as my net is slow at home.