Never, or it will change the answer. (also, PEMDAS is mathmaticly untrue.)
For example, using PEMDAS for 2+1/2, you would divide first to get .5, and get 2.5
Without, it could be 2+1= 3, and then 3/2, and those are diffrent answers.
Though there are exeptions.
Like if its 7(3+x)=42, you don't HAVE to use the distributive property first. You could divide boths sides by 7, and then it would be 3+x=6.