Depends on your definition of "safe". I can see two considerations -
1. Can you successfully carry out the steps to jailbreak, and finish with a working phone rather than a very expensive paperweight. That depends on having good software and your technical ability to follow the process without disasters.
2. Is a jailbroken phone "safe". In the phone world jailbreak is functionally equivalent to running a computer with "root" or "administrator" rights, and we all know this is not the smartest idea when devices are always at risk from malicious attack.
There's another consideration - the phone is certified and approved (FCC, etc) with specific software and configuration. A Jailbroken phone no longer has that configuration and might have unapproved software components so no longer in compliance with approvals. That means a risk the phone may be seized if you attract the wrong kind of attention.
As far as warranty goes, that's a grey area. It would probably depend whether the claimed defect could be blamed on unofficial software.