Unless you expect your water to be very hard, it is not really neccesary. About the TDS meter, TDS measures everything. It is impossible to have less than 100 TDS with hard water, but the other way around is not true; it is possible to have 0 hardness and still a high TDS (from other salts in the water) so it doesn't say much. TDS is more helpful to monitor over time to see whether it gets higher. Also when you know your hardness, TDS can give you a little information about whether there are a lot of other minerals/salts in your water.