Converting ~5V DC to 12V DC safely
5 to 12V step up devices are safe to use. Two things to keep in mind:


efficiency
current


Power efficiency: Say you have a 


1W load at the 12V end and 
say you have a 80% efficiency. 
The 5V end would see a \$\dfrac{1\text{W}}{80\%} = 1.25\text{W}\$ load.


Current: For the same 1W load:


at 12V side you have a current \$I=\dfrac{P}{V}=\dfrac{1\text{W}}{12\text{V}}= \boxed{83\text{mA}}\$ 
and at the 5 volt side \$I=\dfrac{P}{V}=\dfrac{1.25\text{W}}{5\text{V}}= \boxed{250\text{mA}}\$