I'd love to help with your question. I'll never give you advice that I wouldn't give to my own patients or do for my own child.
Although I won't be able to tell you exactly what your daughter has without seeing her in person, I can help frame the decision to start prednisone or not. And based on what you've told me so far, so long as she is fairly comfortable and her symptoms are controlled with what you're doing so far - I'm not sure that you need to start the prednisone. Let me try to explain why.
Basically, prednisone is an anti-inflammatory drug. And it's a very good one: for things like allergic reactions, prednisone is extremely effective. If your daughter is having lots of swelling or itching, a few days of prednisone may be very helpful.
However, prednisone and other steroids have lots of side effects... so I personally try not to use them unless kids really need them. For starters, steroids can cause high blood pressure, poor growth, decreased bone density, and weight gain (in an unappealing pattern especially in the face and neck), as well as a laundry list of other side effects that you may have seen if you Googled them. Generally, these things are only problematic if you use steroids in high doses for a long time, but they are still things for us to be aware of and consider.
While steroids are very helpful for allergic or inflammatory rashes, they will not be so helpful if the rash is caused by an infection like Fifth disease. However, if this is Fifth disease - or some other childhood viral-induced rash - it will run its course and go away no matter what we do. (As a sidenote: I will say that typically, children with Fifth disease have several days of fever preceding their illness... so I'm not sure if that fits with your daughter or not.)
To answer your question about parvovirus testing... I don't think that the testing will be all that helpful, for several reasons. The main reason is that parvovirus in healthy children is a self-limited illness: it will go away on its own, and knowing whether it was parvovirus or not won't change how we treat her. The main reason we send the test is because for pregnant women or children with some chronic illnesses (like sickle cell disease) parvovirus can be a big problem - so diagnosing it specifically is good to know in order to prevent transmission to those kinds of people.
The botXXX XXne, I think, is that if your daughter is overall fairly comfortable with the steroid cream and Benadryl, I don't think you have to start the prednisone. Whether the rash is caused by an allergic reaction or an infection, it will likely get better on its own in the next several days. If, however, the rash is driving her crazy with itching, starting the steroids would probably make the next several days much more pleasant for everyone.
I hope that this information is helpful to you... if you have any questions about anything I've written, please do not hesitate to ask.
Best wishes to you and your daughter!