There are several variables at play when it comes to pitching on a yeast cake. Like the first generation yeast health/viability, sanitation process/practices, fermentation health (originally), remaining viable cell count (pitching rate), % trub, etc...
That being said, I've had success pitching onto yeast cakes in the past, but I prefer to rinse the yeast prior to reuse if time permits.
I don't have a hemocytometer, so I can't count viable cells, but I try to make a best guess for pitching rates. If you start with healthy yeast initially, you'll be better off in the end.