It depends on what system you are using for growing such as soil/soilless vs. Deep Water Culture but for most indoor growing systems a flush is helpful if not essential to optimum plant growth. I have never heard of flushing harming a crop- never heard that from any grower.
Unfortunately, most of us cannot monitor our plants root zone EC/TDS for 24 hours a day. Because of this we do not always know which mineral's and elements are in short supply, which the plants are using and which are present in abundance.
The safest way to deal with this is to have a constant supply of nutrients in the root zone.
The problem with that is inevitably you will have a buildup of nutrient salts and it can become toxic to the plants. The solution is a regular flush with water or a flush solution. We recommend flushing regularly.