It's normally a piping designer or engineer's job. The general considerations are:
a. minimum back pressure or back pressure is considered in control valve sizing.
b. work place safety. people will not be hurt by sudden blow of air or any other fluid.
c. don't let condensate stay in the pipe.
d. if the fluid is liquid, it should get gack to the process or to a drain (for water).
e. if it's steam, consult a thermo piping engineer since you have to consider not only the above a, b, c but also the stress impact on the piping.
It's typically handled by piping contractor. Don't assume he automatically knows what to do. There are guidelines in ASME sec 1 or Sec VIII, depending on whether you are relieving a boiler or an unfired vessel, respectively.  Primarily, keep it short and as straight as possible.  Consider the reaction force when hanging the pipe.  Steam valves discharge thru an elbow with a drip pan on it.  The exhaust stack is a size larger than the ell, and runs vertical to the outside.

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