I might not be the best one to comment on this as I'm currently dealing with some water quality issues that I think is stemming from my RO unit but...
Acceptable for me, would probably be anywhere 5-15 ppm, kind of hard to say as I always run through DI resin for one last cleanup. It's important to note that you're not really measuring one particular atom/element in the water whatever these "solids" are, but instead of are measuring free ions (try to measure your reef tank with lots of free ions, your TDS meter will max out), so who knows what exactly is causing readings on your TDS meter.
That said I've always been of the mindset that the tank (feeding, pooping, etc) will make the water WAY dirtier than the water you put into it, some people like to start with a clean slate when they do it, I don't feel like trying to squeeze those last few PPM out of your fresh water will make that much of a difference in the long run. Now if your putting in water straight from the tap with some chloramine absorbing thing in it and your TDS is north of 100 you might run into issues... might, you could also have no problems if everything in your tank is in a nice sync with one another, just enough algae to consume whatever nasties are in the water, and just enough larger critters to consume the algae that's eating the nasties, etc.