I didn't experience tiredness when I tapered off of Prednisone, but I believe that is due to the fact that I tapered off of Prednisone due to the fact that it didn't work for me and had started taking Dexamethaone, another steroid, several months before I fully tapered off of Prednisone.
When Dex, which I have been on for six months now (got to love steroids...not), I take doses for four straight days and then I have 10 days without any doses. Actually, I just started having 17 days without any doses this past cycle. The second day of the not taking any doses (so one full day after the last day of my pulse), I call my crash day. I am so exhausted, even after sleeping 10 hours at night, that I will commonly spend the entire day in bed, sleeping. It is not uncommon for me to sleep 20 of the 24 hours of the crash day. It was so bad that I was taking the day off of work, because I couldn't keep my eyes open to function. Thankfully, I was able to switch the days in which I pulse so my crash day now falls on a Saturday. On a regular day, my body only needs about 6 hours of good sleep at night, and I'm wide awake for the entire day. I've never been the type of person who would sleep until noon on a Saturday.
When I talked to my Hemo about this, he called it withdrawal. It was my body's natural reaction to not having a new dose of the steroid in my system. Perhaps this is what you are experiencing with Prednisone? If it helps, it is something that I fully expect will go away once your body adjusts to not being on steroids anymore.
On a related note, steroids (neither Prednisone or Dex) did not help my ITP much -- I'm in the process of tapering of Dex to be done with it due to a relatively unsuccessful treatment -- but they really affected me in other ways. I experienced a ton of side effects, and my family used to joke that they could tell which days I took Dex and which days I didn't (even if they hadn't memorized my pulse schedule), because I was incredibly hyped up on Dex. I had tons of energy, talked a mile a minute, and couldn't seem to shut my brain off. Actually, when I was taking a higher dose of Dex, I used to have to take sleeping pills in order to be able to sleep on the nights that I was on my pulse. Otherwise, I probably wouldn't have been wide awake for all four days of the pulse.
Anyways, after my very long post, you are not alone with the feeling of tiredness. In fact, through looking around these boards and through my own experience, I believe that ITP does impact the energy levels of a lot of people. Ever since I have been diagnosed, I have definitely noticed a decrease in my energy level. I'm not sure if that is a direct result of the ITP or a result of all the medications I have been taking to treat the ITP. For someone who never took pills for anything (beyond the very occasional Ibuprofen -- not allowed anymore), the massive amount of medication I have been on since my diagnosis seven months ago has been a really hard adjustment -- emotionally and physically.