I've done it at every watering in my outdoor with great success. Can also do feed-water-feed if you want. If the plants aren't showing signs of deficiency there's no need to feed at every water. Being able to read what the plant wants is a big part of success with growing.
I do EWC tea, and they would love it if I gave it to them every time, but I'm doing it less and less since it's a little extra work to clean the equipment after.