One of the problems with some tennancy cleans is that any cleaning usually comes out of the tenants deposit, so there may be limited funds available. If you are going in at an hourly rate you would normally be expected to state how many hours you intend to be there, I would be very surprised if they gave you an open ticket to clean at your leisure, if they did then you would certainly make more money.
If you had a contract where you charged a certain amount of money regardless of the condition of the property then you would not need to view the clean, this may work in your favour over a period of time as some properties would not need as much cleaning as others, but it would have to be at a rate that would ensure that the bad ones did not matter. I have seen this work with house reposessions, company would charge a fixed price to clean, clear, change locks, drain down the water system etc.
I personally would not take on a job unless I had seen it first as you could be letting yourself in for all sorts of problems.
Hope this helps
Derek