I don't know what you mean by common practice, I don't see it too often that I'm aware of. However it is not illegal to do so. I've run some larger scale plans through a copier that was capable of doing so without being concerned. IN fact if you buy a set of plans you will have to make copies for cutting parts. If you copy then sell them outright to others especially online, you may be in for some trouble, especially from major plans makers such as Ziroli, Hostetler, etc. Kit manufacturers might also get a bit uptight.
As long as you do it for a friend or even for yourself as you may want to have an extra set in case you have an unexpected landing, there should be no problem. I keep all my plans for future use anyway.
So if someone asks to make a copy of a set of plans as a favor you shouldn't have to worry about getting into trouble.