Hi Northerner,
I don't know about cutting back petunias and if there will be time for a setting of seed from regrowth at this time. I let my petunias grow and do nothing, lol, so I don't have trouble finding seed. I do know each seedpod has quite a few seeds. So maybe you could let some plant growth stay for seed saving and prune back the plants in more noticeable locations if you wish to make them look nicer.

For zinnias, there are no seed pods. You let the flowers die on the plant until they get very dry. At the base of what used to be the flower will be arrowhead shaped things. They are often still attached to what used to be the flower. Those are the seeds. Ones that are solid are good. Very thin bendy ones are not.
Remy

Thanks for your help, Remy. The reason I haven't let these petunias 'just grow' is because they are in pots hanging on my deck rail. Some years ago, Mark Cullen said that when your petunias get ratty looking, just give them a good hair cut and that would rejuvinate them. So I wanted to do this, but these are very precious and was afraid to lose all the seed. Anyway, I will hold on and see what I can get. The zinnias have just started blooming well, so it'll be sometime before I can start collecting them. Thanks again.
Northerner.