5 mg is very common, I think.
Because we had tried so many times and failed, we consulted a reproductive immunologist. One of the things she had us do on our next cycle was up the dosage to 15 mg. On the 2nd try after that, I had a successful pregnancy. (5th donor, 16 DE embryos transferred)
I know that for some illnesses, people have to take much higher doses of prednisone and that it can be bad then, but I believe there are indications that it can be helpful in low doses when ttc.
If you take it, do take extra calcium and vitamin D when you take it as it sucks the calcium off your bones, and wean yourself off of it slowly, cutting the dose in half for a few days, etc. before going off completely. Oh, and my RI was very strict about making patients take the whole amount IN THE MORNING and not later in the day.
Good luck to you.