Probably not every time for adult frogs, more like every other time or every third time.
Edit: It's easier to dust if you have a small container with a lid, just put some vitamin powder in, put the bugs in and shake.
Edit Edit: With some calcium supplements you need to space them out. If you have a separate vitamin supplement you want to give it's best to give it on a different day than the ones you give the calcium on. However, the Repashy Plus stuff I have is designed to be given with every feeding.