You can just do it by the numbers, but this leaves open the possibility of a less than optimal fit. It's an example of tolerance stack.

For a certain class of thread fit, there is a tolerance specified. If the can happened to be at the high end of the tolerance, and the barrel happened to be at the low end of the tolerance range, you could end up with some slop, even though everything meets specs. Having the suppressor there eliminates that possibility.

It's not required, but it's nice to have.