It will not hurt a thing. It can be used on occasion to align certain flash hiders and comps that you can not get lined up properly. And you do not have extra Crush Washers to see if one will work. Or we do not want to modify a washer. As stated if needed will not hurt anything. As with any barrel work just be sure you recheck your Zero due to possible changes and effects on barrel harmonics.
The rule on using a crush washer for a flash hider for example is if you must turn it 3/4 turn or more to get it into position you should try another crush washer due to slight variances in thickness dimensions in them. And not use them if you only have to turn them less than a quarter of a turn to get them in place. When you install them it is exactly as stated. You crush them a little bit, back them off "only until the pressure is relieved" then crush them and repeat until you get them into position. You also only want to use them once unless you hate the guy who is going to work on it again or if you are going to plan on changing it again. If you use one twice you will more than likely have to cut it off the next time without hitting the threads or the finish on the barrel. Slightly a tedious task! Not recommended!