After 9 things are calming down, by 9.30 things are pretty good. You won't get a seat at either time, unless you are very lucky, but from 9am should get on a tube pretty much straight away - at worse waiting for the second one on the platform.
Only if there were delays earlier in the morning will you have a problem and that's v rare these days. Most probs still caused by sheer volume of people rather than anything technical.