It's quite safe - as long as you know what you're doing.
There will generally be some operating tolerance, for example my Celestron power pack reads over 13v with no load, but if the power supply is rated at 12v and is regulated then there's little danger of any problems occurring. The real danger is if someone uses a cheap unregulated plug pack as the no-load voltage on those can easily be several volts higher.
It's also quite common for people to use 12v gel and lead acid batteries and these are often 13.5v when fully charged, further proof that there is "some" margin.