Is it Safe to root a phone that has been flashed to a different carrier?

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My cousin just had his phone Samsung Fascinate flashed to Boost Mobile and I just told him about the magic or rooting your phone. He wants me to do it for him, But I'm unsure if it will screw up his phone.

Can anyone let me know asap?

Normally not, the setting that are changed when flashing a phone to bost (prl, mdn, ...) are on a separate nand for the radio. The only thing that might change is MMS capabilities. However, if you dont change roms and just root the phone you should be fine, but hey, who wants to root a phone and keep stock rom? For the fascinate I would recommend using odin to flash CWM, make a nandroid backup and then root, that way you can always restore the backup if anything goes wrong. Also when using odin DO NOT flash a radio, it probably will undo the flash.

Edit: also, please post in correct section next time /Fascinate/General Q&A/ i'm usually nice about this stuff, but not everybody in the community tolerates miscategorized threads