I've been travelling by bus a lot this month, and I've noticed some changes. In the old days, for example, people used to queue at the bus stop. That seems very last week now. Also, buses used to stop so that the door was near the bus stop sign; now though they stop either 10m beyond it or 10m short. But this question is about alighting, especially from upstairs which, for those who haven't realised, is where the seats are.

For safety, all bus companies recommend remaining seated until the bus stops.
All bus braking differs and varies from rough to less rough!
Very difficult to bring to a stand smoothly unless a slow approach is taken.

Good question ...

I have noted a difference in buses locally and in London (the two places I use them significantly). Locally, as you get on the driver waits for me to sit down before he starts, but in London there is far less waiting for passengers to be seated.    I tend to take that as an indicator for when to get up; locally I usually sit until the bus is pretty much halted but in London I'm used to doing something of a trapeze act down the stairs as e swing around corners.  But on approaching a major stop where I know lots of people will be getting off, I'll tend to wait.