Isolation valves on tankless heater: is this setup ok?
This kind of connection does not need pipe dope because a plastic washer or seal is compressed between two hard metal rims. The threads are not exposed to water pressure and are not part of the sealing so pipe dope would not be useful.

Twelve years ago I bought and supplied to the plumber who installed my tankless heater a set (hot and cold) of Webstone 50443 service and isolation valves. I had bought two of these Bosch heaters as discontinued models on sale at Home Depot, intending to put one in my house and the other in a rental house I owned, but I chickened out on doing it myself and hired a plumber. He acted like he had never seen valves of this type. These are specifically for tankless heaters and allow cutting off the cold water supply and the hot water piping of the house from the water heater and then opening up the heater for draining and for descaling with vinegar.

I had it descaled once in 12 years of service. The plumber who did the descaling was not familiar with these valves and wrenched one in the wrong direction over the stop. Luckily this didn't break it and I was able to remove the handle and restore normal operation. He used a pump in a bucket to circulate a couple of gallons of white vinegar but  decided to turn the heater on with the vinegar circulating. I don't think that is a good idea. 

I wonder whether descaling is necessary for most installations. If yours is new just use it for 10 years and then think about descaling.

What model is your tankless water heater? Mine is a Bosch Aquastar 125 (?), a discontinued model with a standing pilot light that does not require electric power.

Noritz seems to think that there is no one universal descaling time interval. http://www.noritz.com/blog/biggest-myth-tankless-water-heater/