'tis wise to be leary, but it is commonly practiced, even to the point of flame cutting the bolts while the line is under pressure.
The thinking is that if the pipe is not too hot, it is okay...
but you have to look at the piping class, temperature (personnel protection), lethality class, plant operations, etc. to better guage that. When you are completely satisfied with you draft, you go to a different area of the plant and you find that the welder will not touch a natual gas line unless it has gas in it...with the flames pouring our of the joint while being welded...
are you sure the piping forum is the right place for the question?