No its not safe but the risk is yours and depends on unlock method
Do you mean jailbroken (to install applications etc) or unlocked (to enable use on any network)? If the former than you probably OK but if the second......
As you don't know the method used then you may end up with a bricked phone - if you use either the iPhoneSIMFree method then you'll be OK (posted on their site) it works even with phones which are bricked or unlocked 'poorly'. I have tested this and it works even with a bricked phone. If you follow the procedures for free relock/unlock then there is a risk of a brick (check Dev team sites, both Goggle Code and Wiki) - I have not tested this method so consider it a risk. If you just update with the free method then chances are you'll brick the phone.
I would assume the worst and use either iPhoneSIMFree (it will cost you $60) or the free method (I have not tested the latter myself so can assess the ease or risk)