There isn't a level of PPM for tap water i would say is bad but it really depends on what is making up those PPMs that could harm your plants IMO.
the water where i live comes out at 480ppm and its VERY hard (lots of calcium and mg), BUT i still hit great numbers. You can get a water analysis from your water company and that will tell you what is in your water. but i would at least try it first and save yourself the money. if you start seeing problems, then investigate a little and see if an RO sys will do the job. I got an RO now and will say that things just seem to be all around improved, but by no means was it the magic sauce that all the sudden made my yield double (that would have been awesome though).