PBS Help
This has all the answers you need, especially the PBS Gouge--page 132 specifically, "Reserve Line Construction Rules." It all depends on whether or not you have work days or off days at the end of the previous month. If you have a trip (or on call days if the previous month was also reserve) ending on the last day of the month, then you can "orphan" the days as you said, so long as by doing so you meet the "min days on" criteria.
If you do not, then you cannot do that. The Gouge explains it very well. There is also an Excel spreadsheet on the site, "Reserve Line Simulator" which will actually tell you if your desired reserve schedule is legal or not contractually.