Although v26 apps should work well in iOS 7, they haven't been tested as thoroughly as v27 apps. When iOS 7 releases tomorrow, test your app to make sure that it works.
If you have a v24 or earlier app, you should update it to v27 or v28.
I tested a few of my v26 apps and no problems, but doesn't mean too much, other than to give you a bit of confidence that you should be ok.
Question to Bob; are there any specific things that you know iOS7 doesn't like from v26?
Here is a list of bugs discovered in v26 viewers on iOS 7 and fixed in v27/v28:
There's no need to update those older folios unless you want to take advantage of newer features that require an updated folio format. In other words, it's the app itself that needs to be updated, not the folios.