The more accurate your FTP is, the better the system can train you, so doing an FTP check every couple of weeks is ideal.  Not only that, the FTP check is a great workout and it can lead you to a higher FTP, which is one of the objectives of training.  

If you do hard 20 minute efforts outside of the FTP tests you can mark those as FTP checks and ignore the FTP check in your plan that week.  

Of course, if you don't want to do an FTP check when it's assigned, you don't need to do it.  The plan will adapt.