Amazing how opinions have changed over the years, 5 years back there were guys on this forum pulling their hair out in a panic when their tds reached 002
I think you summed it up on a previous comment - it has much to do with the composition of the water in a particular area and that area's weather.
I won't let ours go above 2, period. We can get away with 6 just, but anything above that I can see fine spots on our test windows. We also live on the North East coastline so need the water to be as pure as possible. We have to spend more time washing and rinsing in bad weather, so why add to that time further with not so pure water.
When we first started Peter Fogwill told me that we could safetly get away with a TDS of 4 and that it what we have found in practice. I helped a cleaner with work a few years back when his van was in for engine repairs. He works inland and a noticed how much easier his daily work is away from the coast when compared to ours. So he could maybe get away with a higher TDS - but I wouldn't chance it.
I personally believe it's up to each of us to keep our standards high. After all, its our reputation that is at stake. I did a commercial clean yesterday and I have been told the boss checks the quality of the clean on random windows after we have been. We also clean the family home and he does the same.