Using rain water...

If your pH tests fine, there should be an issue. Water vapor is more or less pure until it condenses. Rain requires condensation nuclei (Hydrogen sulfate, Saharan dust, factory soot, sea salt etc.) to in order to condense around and form. Using a filter rated for micrometer sized particles or greater should rid you of those particles.

There are so many unknowns and quality will very greatly depending on locations. I wouldn't use it.

 

tds is just one aspect of the water. Copper and other pollutents can wreck havoc on a reef tank. IMO it isnt worth the risk

 

I second this. There are simply way to many potential contaminants to take the risk.