We hopped on and off the tube all the time when we were in London last October. We never had a problem.
I'll admit I had dh to help me up and down the stairs but there were lots of occasions where people thought I was on my own and offered to help me up/down the stairs, which was lovely. We did try at all times to avoid rush hour though, but on the odd occasion where we misjudged it and got caught it was fine.