As long as the controller you are working with agree, any altitude is OK.
Eastbound, VFR at 5,500, with flight Following, I encountered a severe, and continuing updraft. Yes, that can happen.
Called my controller, and advised that I was at idle power, and rising out of my reported altitude. He responded "I suggest that since when you fly out of that updraft, you may encounter a strong down draft, exceeding your climb capability. If you resume cruise power, altitude at pilots discretion up to 12,000 feet"
I acknowledged affirmatively, and soon found myself at about 8,000, but not climbing.
I reported this to ATC, and he said "Maintain level flight until I give you an assigned altitude".
10 or 15 minutes later, we requested a cruise descent to college Park MD, and then began a steady slow reduction to pattern altitude.
Key points here...both the controller and I knew what I was going to be doing, and agreed.
The controller was in fact monitoring traffic to assure that I was not going to meet anybody.
Due to the presence of the two towering thunderstorms that I had passed well clear of, and between, no other VFR traffic was up there. I had blue sky above me the whole flight, good visibility, and no turbulence, just that strong smooth updraft.
I strongly suspect that my controller was a GA pilot. He certainly thought like one. I have heartily criticized a few select controllers, but his one was one of many fine examples of the best in the business of keeping us safe.