I disagree. You may not HAVE to change it out, but I feel you should. You have no way of knowing exactly what the solution content is. Is there more nitrogen? More calcium? Enough of either? Enough magnesium? It doesn't matter if your EC is 1.8 if you don't know what content is making it 1.8. Not all elements have the same level of conductivity. EC is only a guide that is useful when you do regular changes.
nutrients need to be somewhat balanced in order for optimum uptake. That isn't to say they won't uptake at all. Thing is, most people that think their plants are doing great probably don't realize how much greater they could be doing if they only fed the plants better. I've learned this from trial and error. My error being greatest when I let others convince me that regular changes are unnecessary. I've run side by side experiments using GH Flora series nutes. Regular changes definitely produce much greater yields and require a lot less daily maintenance of pH.
I think some people get it in their minds that if their hydro plants look like garden plants, then they are doing great. Well, they are doing okay. Hydro plants should look better and produce more than the soil garden.
Keep in mind it isn't just about what nutes aren't in there. Toxic salt build up can occur. It might not kill the plant, but it can reduce yield by reducing uptake of nutes and water. It's easy to see when your plants start looking a little crappy and we get questions on other forums "Why am I getting BER" or "Why do my leaves have yellow between the veins" and such. And the results of the problem are usually not seen immediately, as is rarely the case with toxicity or deficiency. People misunderstand that just because they made a change this week and the problem arose this week then it must be due to the change. Not so. The problem could just now be evident from the previous reservoir imbalance. Another problem that can occur from toxicity and deficiency is greater susceptibility to disease. Plants can fight off a lot when fed well. Just like us, if they aren't they can get sick easier.
One last point: there is a reason the formulas are fairly precise. There is a reason that when people make their own nutrients it is important to make sure you maintain some precision. This haphazard way of things completely contradicts this. I don't do hydro so my plants can do okay. I do it for several reasons, but the main is for the challenge of growing the best my plants can possibly offer. Just as our bodies do best when fed right, so do my plants. How much money are you wasting feeding plants that could be producing better? Is it more of a waste to change the solution regularly or to risk deficiency or toxicity and get less yield?