With all the net groups springing up on every band it's becoming too easy to QSY to a frequency that a net runs on, only to find after a few minutes that the net is due to start. I know that there is no such thing as reserved frequencies and that a free frequency is a legitimate one for any QSO to take place.
I do think however that nets should not be run on 20m as this is the most used band for CQ QSO's having the 14.200 calling frequency. Perhaps it would be better for both network groups and CQ callers if all nets were confined to one band? It is certainly unreasonable to have to check what frequencies are in use by net groups before having to QSY ON 20M. Life would be a lot simpler if it were just a matter of avoiding one band.